Option A is the correct answer because the sentence is an exclamatory sentence that expresses surprise, astonishment, or any other such strong emotion. It is a rule that we use an exclamation mark at the end of such a sentence.
Option B is incorrect because a question mark is used only at the end of a question, however, the sentence is an exclamatory sentence and not a question.
Option C is incorrect because a comma is used to indicate a pause and we expect the sentence to continue after the comma. Hence using a comma at the end of this sentence would make it incomplete.
Option D is incorrect because an exclamation mark is required at the end of this sentence to make it complete.