Hello Diamond, I would say "either" too. You might say that "gather one's thoughts" often relates to "preparation for X", whereas "collect one's thoughts" often relates to "recovery from X"; but in that case, C would be the answer, not A. So that presumably was not the intention of the question-setter.
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DiamondrgThank you, MrPedantic and Feebs.I hadn't seen Mr P's answer (we hit the button at the same moment). On reading, I am going to stand by my "A".
PS: Hi, Feebs. A or C fits best? I think MrP thinks C, not A, fits best.