Hi sbose!
Both the stimulus and each of the answer choices you mentioned discuss two different people (Keeler & Greene, Carter & Whitequill, and Helms & Lapinski), so this is not a reason to eliminate answer choice (C).
When comparing (B) and (C), one of the biggest reasons to eliminate (B) is because this answer choice discusses both people having a reason (motive) to do something then opting NOT to do that out of fear, while the stimulus discusses one person having a reason (motive) to do something and another person having NO reason to do something.
If we were to break down the stimulus more abstractly, we'd get something like this:
A (Keeler) has motive to do something.
A and B (Greene) were the only ones with the opportunity to do that thing.
B does not have motive to do something.
Therefore, A did it.
This is exactly what we get in answer choice (C):
A (Helms) and B (Lapinski) are the only ones with opportunity to do something.
A had reason to do that thing.
B did not have reason to do that thing.
Therefore, A did it.
To contrast this, let's look at (B):
A (Carter) and B (Whitequill) are the only ones with motive to do something.
B is too afraid to do that thing.
Therefore, A did it.
As you can see, (C) matches the stimulus much more closely than (B) does! Since this is a Parallel Flaw question, we need to parallel every aspect of the stimulus, including the Flaw itself. This means that the language should be the same (or logically equivalent), the premises should be the same, the reasoning should be the same, the conclusion should be the same, and the abstraction should be the same. The topic, however, does not need to be the same.
I hope this helps!
Kate