Hi, there. I'll give my 2¢ here.

First of all, where did you get this question? Do we have any guarantee that the answer (or source) can be trusted? Also, are you sure you didn't miscopy any of the numbers?

What I can tell you is: mathematically, your answer of 12.3% would appear correct. It is certainly true that, of the five categories listed, the fellows constitute 12.3%.

The only thing I can think is --- perhaps in the mind of whoever wrote the question, "associate members" and/or "affiliates" don't count as official members. (That would be much more of a picayune semantic trick than anything mathematical --- the real GMAT would not play such games.)

If we go ahead and say that both "associate members" and "affiliates" don't count as official members, then we have that fellows are 20.56% of the members ---- closer to the OA given, about which I have substantial doubts.

I don't really know what to say about this question --- either the OA is wrong, or it was miscopied, and it may even involve some subtle semantic games about who exactly would count as an official member. All and all, unless it's radically misrepresented here, I would not consider this a high quality question at all.

Here's a considerably better, considerably more GMAT-like PS questions about percents:

https://gmat.magoosh.com/questions/29

When you submit your answer to that question, the next page will have a complete video explanation.

Let me know if you have any questions on what I've said. Best of luck in your studies.

Mike