Hello @divineacclivity,

You post a very interesting question. I can understand why you thought that 'by' could mean 'beside' in this context, since that is one of the meanings of 'by'.

The friends gathered by the fireplace to chitchat.

However, this meaning of "by" is not the meaning that is applicable in this context. Remember that meaning is context-based, and to determine the appropriate meaning you should take your clues from what the original sentence tells you.

So first and foremost, let's understand the meaning of the original sentence

Sound can travel through water for enormous distances, prevented from dissipating its acoustic energy as a result of boundaries in the ocean created by water layers of different temperatures and densities.

This sentence is not written in a manner to communicate the meaning effectively. But we should review the sentence carefully and ascertain the implied meaning.

1. This sentence first states a fact that sound can travel through water for long distances.
2. Then it states that the acoustic energy of sound is prevented from getting dissipated because of certain boundaries in the ocean.
o These boundaries are created by water layers that have different properties (temperature and densities)

Now what the sentence fails to establish properly is how these two facts are connected to each other.

So let's connect the two:
"¢ Clearly, it is logical to say that because of fact 2, fact 1 takes place.
o That is because the acoustic energy of sound is not dissipated easily, sound can travel through water for long distances.

So this is the meaning that the correct sentence should communicate.

Now with this understanding let's look at what choice C does:

Sound can travel through water for enormous distances, its acoustic energy prevented from dissipating by boundaries in the ocean created by water layers of different temperatures and densities.

In this choice the incorrect part of sentence "prevented from..." has been converted into a modifier of the structure noun phrase + noun modifier. Here the noun phrase is "its acoustic energy". This modifier modifies the preceding clause, providing additional information about the action in preceding clause. Essentially this modifier answers the question - Why can sound travel through water for enormous distances?

Thus this sentence communicates the intended meaning of the sentence.

Now before moving to Choice D, I would like to address your doubt about the meaning communicated by "by". As you can see, the context of the sentence requires the meaning of "by" to be "by means of" and not "besides". So always remember to thoroughly understand the context of the original sentence to avoid making such errors. Do let me know if you have any questions about this.

Now coming to choice D.

Sound can travel through water for enormous distances, its acoustic energy prevented from being dissipated as a result of boundaries in the ocean created by water layers of different temperatures and densities.

This choice is incorrect because it has meaning ambiguity. Is "as a result of" referring to "prevented" or to "being dissipated"? I.e. which of the following meanings is the choice communicating:
"¢ Acoustic energy is prevented from being dissipated.
o This dissipation happens because of boundaries in the ocean
"¢ Acoustic energy is prevent from being dissipated
o This prevention happens because of boundaries in the ocean

Concluding comments
So as I always say, read and thoroughly understand the meaning of the original sentence. Once you do that, answer choice analysis becomes pretty straight forward.

I hope this helps. Let me know if you have any other doubts.

Regards,
Meghna