Because there's a "but" after "thin", the word after the "but" can't be a synonym or something generally suggesting the same concept so "wiry" and "stringy" are definitely not the correct answers. Despite "worried" not being a synonym of "thin", it is not fluent and does not make sense to say "thin but worried" as the "but" suggests something generally opposite or different whilst "worried" has nothing to do with "thin".
A. wiry Thin and wiry are not synonyms. They are related concepts. “Thin but wiry” means thin but possessing strength as shown by visible muscle fibre. Wiry is the correct answer. But does require something other than a synonym, but it can’t be a completely unrelated notion. Wary and worried are unrelated to physical appearance. Thin but stringy does not provide any new information to refine Thin. Wiry means lean but has the additional sense of possessing strength “tough” “sinewy” that stringy does not. Wiry is the correct answer. Thin, wiry… are frequently used together in physical descriptions of people or animals when apt.
