The correct option is A stars
An idiomatic expression is a type of informal English where the meaning
of the expression is different than the literal meaning of the
expression.
Option A: After placing the word 'stars' in the blank, the phrase formed 'reach for the stars' is an idiom; it means 'to set one's goals or ambitions very high', as high as the stars. It relates to the sentence: He is really ambitious. Hence option A is correct.
The words sun, clouds and roof, each do not form an idiom when placed in the blank. Nor do they provide the required meaning, which is expressed by the idiom with 'stars'. The meanings formed are quite literal and irrelevant. Hence options B, C and D are incorrect.