The correct answer is: D. no article.
The word “chairman” is a common noun, which means that it does not refer to a specific person or thing. In this case, the speaker is not referring to a particular chairman, but rather to the position of chairman in general. Therefore, no article is needed.
If the speaker were referring to a specific chairman, they would use an article. For example, they might say “He was elected the chairman of the board of directors.” In this case, the article “the” is used to identify the specific person who was elected chairman.
The article “a” is used before singular common nouns that refer to a general class of people or things. For example, you might say “I’m looking for a new job.” In this case, the article “a” is used to indicate that you are looking for any job, not a specific job.
The article “an” is used before singular common nouns that begin with a vowel sound. For example, you might say “I’m looking for an apartment.” In this case, the article “an” is used because the word “apartment” begins with a vowel sound.
I hope this explanation is helpful!