He returned after . . . . . . . . hour. A. a B. an C. the D. no article

The correct answer is D. no article.

The word “hour” is a non-count noun, which means it does not have a plural form and does not take a determiner (such as “a”, “an”, or “the”).

In this sentence, “hour” is used to refer to a general period of time, not a specific hour. Therefore, no article is needed.

Here are some other examples of non-count nouns:

• water
• sand
• air
• love
• happiness

Note that some non-count nouns can be used with a determiner when they are used in a specific context. For example, you could say “I need a glass of water” or “I love the beach.” However, in this sentence, “hour” is not being used in a specific context, so no article is needed.