In this sentence a man's inability to open the door due to weakness is emphasized. The option much (adv.) is used to emphasize a comparative adjective. Whereas too is used to modify an adjective creating the meaning of an unacceptable, excessive degree. Option very is used in cases to intensify an adjective .far is again used where comparative degree of adjectives are used. Here the sentence highlights the inability of the man due to excessive weakness. Thus it needs an adverb to highlight the severity of the problem. Thus option (d) is correct.