Answer: - b) Likely.
Detailed answer: -
I don't think the boss is very likely to agree.
In the sentence very is used so with the word vary likely is suitable to use.
Likely is used to refer or indicate the chance that something will happen.
The adverb 'likely' is mostly used with very and or used by itself in a sentence.
The word 'likely' is an adverb.
The word 'probable' also means that something is likely to happen. But it is not suitable to use with vary.
The word 'suitable' means something which is right for a person or situation.
The word 'talkative' means a person who talks a lot.