ironman_97I posted about that here (
https://gmatclub.com/forum/i-am-confuse ... 32791.html ), but I've copied my response below for convenience, since it doesn't depend on the content of the original post . . .
If "one" is the subject for the verb, then we use a singular verb. However, if one is modified by a plural noun that has its own verb, then we use a plural verb. The problem is that it's not always clear what's actually happening. Consider this pair:
One of my friends is sick.
One of my friends who lives in Philadelphia is sick.
In both cases, it's clear that only one of my friends is sick, so "is" is clearly correct. But what about "lives"? Perhaps I have more than one friend in Philadelphia, so could I say "One of my friends who live in Philadelphia is sick." Logically, this seems okay, but I don't think a fluent speaker of English would ever produce this sentence. We'd always keep "who lives" modifying the whole noun phrase "one of my friends" and therefore use the singular.
There are many other cases in which we generally avoid the plural, even though it works in theory. This usage is simply idiomatic:
This is one of the things that bothers me about warm weather. (We wouldn't usually say "This is one of the things that bother me," even though that actually makes more sense. Hey, I never said English was logical!

)
Timing is one of the factors that really makes a difference in your performance. (We might use "make" here, which would bring us close to the originally-posted example, but it's clear either way.)
So when do we use the plural? When we must! Typically, this is when we are clearly talking about all the elements that the "one" is pulled from.
The guitar part is only one of the many elements that comprise the composition. Here, the emphasis is not on the guitar part, but on all of the elements, so the singular doesn't work.
The liver is one of the organs that protect your body. Again, we are stressing that one thing is part of a larger group, while in the earlier examples we were emphasizing the importance of one particular item in a group.
Conclusion? This is an area where English is complicated and sometimes illogical. How will the GMAT handle that? The way it always does--by giving us only one answer that works! We're unlikely to see two answers that hinge on this issue alone. If that does happen, then we have no choice but to default to logic and make sure we know the proper subject for the verb in question. Otherwise, be ready to be flexible and look for more concrete reasons to eliminate each choice!