The correct answer is: Everybody was surprised at what had happened.
The sentence is in the past tense, so we need to use a past tense verb. The verb “surprise” can be transitive or intransitive. When it is transitive, it takes a direct object. In this case, the direct object is “what had happened.” When it is intransitive, it does not take a direct object.
In this sentence, “surprised” is being used in the transitive sense. We can tell this because there is a direct object, “what had happened.” The past tense of the transitive verb “surprise” is “surprised.”
The verb “happen” is also in the past tense. This is because the sentence is in the past tense. The past tense of the verb “happen” is “happened.”
Therefore, the correct answer is: Everybody was surprised at what had happened.
Here is a brief explanation of each option:
- Option A: Everybody was surprised at what has happened. This is incorrect because the verb “happen” is in the present tense. The sentence is in the past tense, so we need to use a past tense verb.
- Option B: Everybody was surprised at what had happened. This is the correct answer. The verb “happen” is in the past tense, and the sentence is in the past tense.
- Option C: Everybody is surprised at what happen. This is incorrect because the verb “happen” is in the present tense. The sentence is in the past tense, so we need to use a past tense verb.
- Option D: Surprise/happen. This is incorrect because it is not a complete sentence.