ans is A.
William Sydney Porter, who began writing while he was in prison for embezzlement, was seeking anonymity when he chose the pseudonym O. Henry.
(B) was seeking that he might be anonymous when he chose
(C) was seeking for anonymity and therefore chose
(D) sought anonymity in the choosing of
(E) sought to be anonymous in that he chose
BCDE all look and sound wrong
B changes meaning
C "for" is extra here.
E "to be" is extra......sought anonymity ....sounds better
I'm crossing the bridge.........