Correct answer:
a. sillón
Un sillón translates into an armchair that is a large comfortable chair with parts for you to rest your arms on. On the other hand, un is the masculine singular indefinite article. The indefinite article is used to refer to a particular person or thing when the reader or listener doesn't know which one is being described, that is, when talking about something that is not defined. Finally:
La sala es pequeña; sólo tiene un televisor y un sillón (The room is small; there is only one television and an armchair)