The sentence "L'homme est _ medecin" requires an article before the profession noun medecin. The noun medecin (doctor) is masculine singular, so the correct indefinite article is un.
In standard French, professions used with etre can appear without an article ("Il est medecin"), but when the sentence uses a noun subject with a definite article (L'homme), the predicate noun typically takes an indefinite article: "L'homme est un medecin."
The other options are incorrect:
une is the feminine indefinite article. It would be used with a feminine profession noun (e.g., une infirmiere), but medecin is masculine.
le is the masculine definite article, but saying "L'homme est le medecin" would mean "The man is the (specific) doctor," which requires a context identifying which doctor. The sentence as written introduces the profession generally.
en is a preposition/pronoun and does not function as an article before a noun in this construction.
