The correct answer is D. no article.
The sentence is about a specific Monday, so we do not need an article. If we were talking about any Monday, we would use the indefinite article "a" or "an." For example, "I do not forget a Monday when we had an accident."
The definite article "the" is used to refer to a specific person, place, thing, or idea that has already been mentioned or is clearly understood from the context. In this sentence, the Monday has not been mentioned before, so we do not use "the."
I hope this helps!
