e-GMAT wrote:Good job @atulmangal for explaining the question posted by catseye.
@catseye, I read through your choice analysis, and I feel that your reasoning is not exactly sound.

Consider the following sentence that has similar construction. Now see if your reasoning applies here on OG12#30 sentence as well:

For members of 17th century Ashanti nation in Africa, animal-hide shields with wooden frames were essential items of military equipment, protecting warriors against enemy arrows and spears.

Would you consider this incorrect because "protecting" does not have a subject?
See the spoiler below for explanation:
[spoiler]This verb-ing word is not acting as the verb of the sentence because it is not preceded by is/are/was/were/am.[/spoiler]

Well, you may find detailed approach to the question referenced in your original post below.

Geologists believe that the warning signs for a major earthquake may include sudden fluctuations in local seismic activity, tilting and other deformations of the Earth's crust, changing the measured strain across a fault zone, and varying the electrical properties of underground rocks.

Meaning Analysis
This sentence provides 4 warning signs for a major earthquake. These are:
1. Sudden fluctuations in local seismic activity
2. Titling and other deformations of crust
3. Changes in measured strain
4. Variation in electrical properties of rocks

A key thing to note in this sentence is that the intended meaning conveyed may be little difficult to comprehend because of the construction of elements 3 and 4. Elements 1 and 2 are clearly nouns (note that tilting is a noun here). Elements 3 and 4 are written in verb-ing form, making it appear as though these two are presenting results of the previous clause.

Warning signs may include A and B, changing the strain, and varying the electrical properties...

This cannot be the correct interpretation for the following reasons:
1. The meaning itself is non-sensical. It does not make sense to say that warning signs includes A and B, and this causes change in measured strain and variation in electrical properties.
2. Grammatically, this is incorrect. In the non-underlined portion of the sentence, "may include A and B" is written as "may include A, B" - If a list contains 2 elements, it should be connected properly using "and" in this case.

Error Analysis of Original Sentence
SV - This sentence contains 1 clause with SV pair = Geologists - believe
Verb Tense - Simple present tense is used
Pronoun - N/A
Modifiers - There are multiple modifiers in this sentence. We will focus on the modifiers in the underlined portion of the sentence only.
"¢ Modifiers for strain (element 3)
o Measured - adjective
o Across a fault zone - prepositional phrase
Parallelism - As discussed in detail above in meaning analysis, this sentence has serious parallelism issues.
This sentence provides 4 warning signs for a major earthquake. These are:
1. Sudden fluctuations in local seismic activity - NOUN
2. Titling and other deformations of crust - NOUN
3. Changing the measured strain - VERB-ING FORM
4. Varying the electrical properties of rocks - VERB-ING FORM
Clearly elements 3 & 4 in underlined portion are not parallel to elements 1 & 2. These can be made parallel by converting into noun forms as:
"Changes & variation"

Answer Choice Analysis
1. Choice A - Parallelism Error
2. Choice B - Parallelism Error
3. Choice C - Parallelism Error
4. Choice D - CORRECT
5. Choice E - Meaning
a. Meaning - The original sentence (Choice A) states - changes in measured strain - i.e. the actual strain has changed. This choice however states that measurements of strain changed - i.e. measurements change but we do not know if the actual strain changed or not. There is a very minute difference between these two but it is significant enough and we must keep a keen eye to detect these.
b. Meaning error from use of idiom - Both "variations in" & "variations among" are correct idiomatic constructions. But they convey different meanings. We need to select the appropriate construction to convey the intended meaning of the sentence.
i. "variations in electrical properties of rocks" implies that we are measuring the "electrical properties" and these are different from 1 rock to another.
ii. "variations among the electrical properties of rocks" means that we are talking about multiple electrical properties - say dielectric constant, conductivity, etc. - and there are variations among these. Clearly, this does not make sense here.

Thus, Choice D is correct.

For e-GMAT customers, this concept is covered at length in the Parallelism Concept Files