The correct option is D Passed by
Phrasal verbs are idiomatic expressions, formed using a verb and a preposition or an adverb. They have a meaning different from the literal one.
Option B: 'Passed by' is a phrasal verb that means 'to go past'. With reference to time, it refers to the passing of the hours. This phrase is relevant here, hence option B is correct.
Option A: 'Passed away' is a phrasal verb that is a more polite way of saying that someone died. This isn't relevant here, as time cannot die.
Option C: 'Were passing' is a continuous tense, whereas the simultaneous action "chances of a result diminished" refers to an action that has completed.
Option D: 'Passed' also refers to time that has gone, but when compared with 'passed by', the latter is a more suitable option.
Hence options A, C and D are incorrect.