The correct answer is B. His behaviour astonished me.
The sentence “I was astonished at his behaviour” is in the past tense. This means that the speaker is describing an event that happened in the past. The verb “astonished” is in the simple past tense, which is used to describe events that happened at a specific time in the past. The sentence can be paraphrased as “His behaviour caused me to feel astonishment.”
The other options are incorrect because they do not agree with the tense of the sentence. Option A, “His behaviour is astonishing to me,” is in the present tense. Option C, “His behaviour astonished me,” is also in the simple past tense, but it does not agree with the subject of the sentence. The subject of the sentence is “I,” which is in the first person singular. The verb “astonished” should also be in the first person singular, which is “astonished me.” Option D, “His behaviour had astonished me,” is in the past perfect tense. The past perfect tense is used to describe events that happened before another event in the past. In this sentence, there is no other event in the past that the speaker is describing. Therefore, the past perfect tense is not necessary.