سوال ۳ کنکور ارشد فیزیک ۱۴۰۵

The blank is directly followed by the preposition "on," which signals a verb that collocates with "on." "Focus on" means to concentrate one's attention on something, giving the natural reading "to focus on the part of the problem that troubled him." The other choices do not fit an intransitive "... on" pattern: "solve" is transitive (solve the problem, not *solve on), "blame" takes a direct object first (blame someone / blame something on someone), and "base" also needs a direct object (base something on).