The correct answer is A. had.
The sentence is in the past tense, so we need to use a past tense verb. The verb “had” is the past tense of the verb “to have,” which is a common verb used to describe experiences. In this case, the speaker is describing a past experience of having a great time at Shimla.
The other options are incorrect because they are not in the past tense. The verb “were” is the past tense of the verb “to be,” which is not a common verb used to describe experiences. The verb “been” is the past participle of the verb “to be,” which is not used as a verb in this sentence. The verb “was” is the past tense of the verb “to be,” but it is not the correct verb to use in this sentence because it does not agree with the subject of the sentence. The subject of the sentence is “we,” which is plural, so we need to use a plural verb. The verb “was” is a singular verb, so it is not the correct verb to use in this sentence.
Therefore, the correct answer is A. had.