To answer this question, we need to understand the concept of conditional sentences and verb tenses.
The correct answer is A) Had ended.
The given sentence is in the conditional form, indicating a hypothetical situation. The phrase "if the world had ended" suggests a condition that did not actually happen. In this case, the verb "ended" is in the past participle form ("had ended") to indicate an action that would have taken place in the past, but didn't actually occur.
Option A) Had ended - This option is correct because it correctly uses the past participle form of the verb to indicate a hypothetical situation in the past.
Option B) Have ended - This option is incorrect because it uses the present perfect tense, which is not appropriate for a hypothetical situation in the past.
Option C) Was ending - This option is incorrect because it uses the past continuous tense, which is not appropriate for a hypothetical situation in the past.
Option D) Is ended - This option is incorrect because it uses the present simple tense, which is not appropriate for a hypothetical situation in the past.
Therefore, the correct answer is A) Had ended because it correctly represents the hypothetical situation in the past.