The first thing we should notice is that the answer choices switch between ending with "are" and "is" - this is a straightforward sign that we're dealing with agreement. We need to find the subject that this verb goes with.

Since there is a phrase set off by commas before our verb, we can go ahead and pull it out of the sentence to put our subject and verb closer together:

The peaks of a mountain range, acting like rocks in a streambed, produce ripples in the air flowing over them; the resulting flow pattern, with crests and troughs that remain stationary although the air that forms them is moving rapidly, are known as "standing waves."

The peaks of a mountain range, acting like rocks in a streambed, produce ripples in the air flowing over them; the resulting flow pattern are known as "standing waves."

So we see that "the resulting flow pattern" is our subject. "Pattern" is singular, so we need singular verb "is". We can rule out A, B, and D.

Looking between C and E, we see that the big difference is between the portions set off by commas:

with crests and troughs that remain stationary although the air that forms them is moving rapidly
vs.
with stationary crests and troughs although they are formed by rapidly moving air

The difference here is pretty subtle: both have a contradiction ("although"), but the contradiction in each is slightly different. C tells us that it is surprising that the crests and troughs remain stationary despite being formed by air that is moving rapidly. E tells us that it is surprising that there are stationary crests and troughs despite being formed by rapidly moving air. C compares the difference between the crests and troughs and the air: one is stationary while the other is moving rapidly. E instead compares crests and troughs themselves to the air itself. This comparison is less clear.

So although C is less concise, it's more precise in meaning, which matters more on the GMAT.