The correct answer is D. No article.
The word "matter" can be used as a noun to refer to a topic or subject of discussion. In this case, the sentence is asking us to discuss a topic seriously. Therefore, we do not need to use an article before the word "matter."
The other options are incorrect because they all use articles. The article "a" is used before singular nouns that refer to something that is not specific or unique. The article "an" is used before singular nouns that begin with a vowel sound. The article "the" is used before singular nouns that refer to something that is specific or unique.
In this case, the noun "matter" is not specific or unique, so we do not need to use an article.
