سوال ۳ کنکور ارشد مهندسی برق ۱۴۰۵

The blank is immediately followed by the preposition on, so the verb must be one that collocates with on. Focus on means "to concentrate one's attention on" — the psychologist told him to concentrate on the troubling part of the problem, which fits the meaning perfectly. Solve takes a direct object (solve the problem), not solve on. Blame uses a different structure (blame X on Y, i.e., needs an object before on). Base also needs an object (base X on Y). Only focus on is both grammatically and semantically correct.