The correct answer is: B. were visiting/exploded
The verb tense of the second clause (“the bomb exploded”) must agree with the verb tense of the first clause (“More than two hundred people were visiting the tower”). In this case, both clauses are in the past tense, so the correct answer is “were visiting/exploded.”
The other options are incorrect because they use different verb tenses. Option A (“have visited/was exploding”) uses the present perfect tense in the first clause and the past tense in the second clause. Option C (“visited/has exploded”) uses the simple past tense in the first clause and the present perfect tense in the second clause. Option D (“are visiting/is exploding”) uses the present continuous tense in the first clause and the present tense in the second clause. Option E (“visit/has exploded”) uses the simple present tense in both clauses.
In addition to verb tense, it is also important to consider the meaning of the sentence when choosing the correct answer. In this case, the sentence is about an event that happened in the past. The bomb exploded while people were visiting the tower. Therefore, the correct answer must use the past tense.