Because I hate you, I included this question on the SSC survey:
It's a weird trick question, but I would say B is right. Imagine converting "(" to X and ")" to Y. Then the first answer is XYXY, and the second answer is YXXY. I suppose you could group the parentheses in pairs, in which case the answer would be "both", but in practice few people wanted to say that. Of the 6,000 answers I received, most were either A or B. And one factor had a dramatic effect: age.
This is a big effect. People in their 20s were more than twice as likely to choose B as people their 60s. There's a slight improvement after 70, but I think that's just noise caused by a low sample size in that group.
My first thought was that the younger population on this blog is disproportionately techies, and techies have to work with very finicky parentheses all day. There was indeed a slight tendency for techies to do better on this, but it was a very small part of the effect. Even controlling for that, or limiting the analysis to only non-techies, most of the effect remained.