It's a confusing question.

I think, if dataset has many obs (say 100 or more, actually, Sasuser.Revenue has 142 obs, but we can't think this particular dataset, should be general, right?), using 'ne', SAS needs to process more obs then this specifical one 'IA11200' , so not effienct?

But, on the other hand, if 'IA11200' is the smallest one in the dataset, so where flightid > 'IA11200' is similar to where flightid ne 'IA11200', right? Both answer A and B are not correct? no answer for this question.

The explanation from the answer also confusing, did not mention A either:

An index can improve the efficiency with which SAS is able to access certain observations in a data set. However, an index is not always useful. SAS will not use an index to process subsetting IF statements, or other statements that SAS determines might be more efficiently processed without an index.