Sentence Correction

On the GMAT, we are looking for the most concise and logically correct option. Generally, assuming all other things equal (in terms of the grammatical rules such as Subject-Verb Agreement, Parallelism, Pronoun Errors, etc.), the verb form of a construction is always considered more "concise" than the noun form. The reason for this preference, on the GMAT and in formal English, is that the verb form is the most direct and succinct way to indicate an action. In most cases, using the verb form automatically results in a more precise and active construction, whereas using the noun form leads to unnecessary awkwardness and wordiness.

For instance, let us look at the following example to illustrate this -

Option A - John completed (verb) the task vs
Option B - The completion (noun) of the task was carried out by John

As we can see from the simple example above, using the verb form automatically results in a more precise, elegant, and pleasing construction. This is one of the concepts that we are going to apply to this question.

One of the splits in the question is in the first word of the underlined portion. Options A, B, and C have the first word as 'of', whereas D and E have the first word as 'that'. One of the main distinctions between a preposition - 'of' is a preposition - and 'that' is that (no pun intended ) prepositions are always followed by nouns/noun phrases. Prepositions are always used to connect/compare nouns, hence whatever follows prepositions must necessarily be a noun phrase. On the other hand, conjunctions - e.g., although, even though, because, while, etc. - and 'that' are usually followed by a complete 'noun + verb' construction. This is, incidentally, the same reason that, among other constructions, we prefer 'Because' over 'Because of' on the GMAT - Because will be followed by a 'noun + verb' whereas because of, being a preposition, will be followed by a noun phrase.

For example: Because John (noun) was (verb) sick, he couldn't attend his classes - preferred
Because of John getting sick, he couldn't attend his classes - awkward and avoidable

Therefore, given all other things are equal and the idiomatic construction permits (e.g., words such as 'forbid', 'prohibit', 'allow' cannot be followed by 'that' on the GMAT), then we would want to go with the options containing a 'that' (or a conjunction). Let us go through the options one by one:

A - Being is avoidable and awkward
B - expansion awkward as a noun. The option also illogically suggests that the 'discovery' was of 'glass' itself
C - Being is avoidable and awkward; expansion awkward as a noun
D - it is able to be is unnecessarily wordy - the pronoun 'it' becomes slightly redundant
E - expanded and shaped - two verb forms connected in parallel and no unnecessary wordiness in terms of pronoun iteration. Also, the sentence logically captures the idea that the discovery is that theglass can be shaped and expanded rather than just of 'glass' itself. This is the best answer.

Hence, option E is the most precise, logical, and grammatically correct answer choice. Please let me know in case anything doesn't make sense.