سوال ۳ کنکور ارشد مهندسی مکانیک ۱۴۰۵

The verb must combine with the preposition on to mean to concentrate attention on something: to focus on the part of the problem. focus on is the fixed collocation and fits the meaning perfectly. The other verbs don't work here: solve takes a direct object ("solve the problem", not "solve on"), blame means to hold responsible (and takes "blame ... for/on", wrong sense), and base means to found on ("base something on", needs an object before "on"). So the answer is focus.