This sentence contains a detour road sign, “nonetheless,” so you can expect the first part of the sentence to contrast with the fact that Larkin “maintained a spirited correspondence with a wide circle of friends.” So you might predict that the missing word means something like “withdrawn” or “shy.” Choice (B) matches this prediction: “By nature reclusive, Philip Larkin nonetheless maintained a spirited correspondence with a wide circle of friends.” That makes perfect sense.