The sentence Il est ___ avocat de profession means "He is ___ lawyer by profession." The blank requires an article before avocat (lawyer), a masculine noun beginning with a vowel.
The correct answer is un, the masculine singular indefinite article. The sentence Il est un avocat de profession means "He is a lawyer by profession." The phrase de profession qualifies the noun, making the use of an article appropriate in this construction.
The other options are incorrect:
des is the plural indefinite article. Avocat is singular here, so des does not agree
l' (the elided definite article) would mean "the lawyer," implying a specific, known individual. In a general statement about someone's profession, the indefinite article is more natural
au is the contraction of a + le (at the/to the) and is a preposition, not an article that can fill this position
The complete sentence is: Il est un avocat de profession - He is a lawyer by profession.
