The correct answer is D. no article.
French is a language, and languages are not typically preceded by articles. In fact, using an article with a language can sound strange or even incorrect. For example, it would be odd to say “I speak a French” or “I am learning an English.”
There are a few exceptions to this rule. For example, if you are referring to a specific language course or program, you might use an article. For example, you could say “I am taking a French class” or “I am learning Spanish at the community college.”
Additionally, if you are referring to a language in a very general sense, you might use an article. For example, you could say “French is a Romance language” or “English is a Germanic language.”
However, in most cases, you will not need to use an article when referring to a language.