Answer:
d. An hour
Step-by-step explanation:
The answer a, has the word "ain't" in it, and good english doesn't use that word. In answer b, "he aren't" is improper use of aren't. Answer c, says "a ice cream", but when the word after "a" starts with a vowel , then you write "an" instead of "a". That only leaves answer d, "an hour"