Là, c'est _ oncle, un professeur au lycee.

The sentence Là, c'est _ oncle, un professeur au lycée ("There, that's _ uncle, a teacher at the high school") requires a possessive adjective before the masculine noun oncle.

The correct answer is mon (my, masculine). The sentence reads: Là, c'est mon oncle, un professeur au lycée ("There, that's my uncle, a teacher at the high school"). The speaker is introducing their own uncle.

The other options are incorrect:

• ma is the feminine form of "my" - it cannot be used before the masculine noun oncle. (Note: even before feminine nouns beginning with a vowel, French uses mon instead of ma for phonetic ease, e.g., mon amie.)
• moi is a stressed pronoun ("me"), not a possessive adjective. It cannot modify a noun directly.
• son means "his" or "her" and is grammatically valid, but in the context of introducing someone at a specific location (Là, c'est...), the first-person possessive mon is the natural and expected choice.