We live on . . . . . . . . earth. A. a B. an C. the D. no article

The correct answer is D. no article.

The word “earth” is a common noun, which means that it refers to a general class of things. In this case, the class of things is planets. Planets are not unique, so we do not use an article with them.

If we were to use an article, it would imply that we are talking about a specific planet. For example, we might say “We

However, when we are talking about the planet in general, we do not use an article. We simply say “We live on Earth.” This is because the word “Earth” is a common noun that refers to a general class of things.