The correct answer is A. ate.
The verb “ate” is the past tense of the verb “eat”. In this sentence, the verb “ate” is used to describe an action that happened in the past. Shantanu ate all the chocolates. This means that he consumed all of the chocolates at some point in the past.
The other options are not correct because they do not describe an action that happened in the past. The verb “eating” is the present participle of the verb “eat”. It is used to describe an action that is happening now or that happens repeatedly. The verb “eaten” is the past participle of the verb “eat”. It is used to describe an action that happened in the past and has a result that is still true in the present. The verb “have eaten” is the present perfect tense of the verb “eat”. It is used to describe an action that happened in the past and has a result that is still true in the present.
In this sentence, the verb “ate” is the most appropriate choice because it describes an action that happened in the past.