Hello!
Answer:
The correct answer is "sa".
Step-by-step explanation:
Cousine is "un nom féminin singulier"
- "Sa" is "un déterminant possessif feminin singulier"
⇒ It's "sa" because you say "je suis la cousine de quelqu'un" and its "au féminin singulier", like the word "cousine".
- "Ton" is "un déterminant possessif masculin singulier"
⇒ It's not "ton" because it's "masculin" and not "feminin".
- "Un" is "un déterminant indéfini masculin singulier"
⇒ It's not "un" because it's "masculin" and not "feminin".
- "La" is "un déterminant défini féminin singulier"
⇒ It's not "la" because you can't say "je suis la cousine" in general. You could say "je suis la cousine de quelqu'un".
Conclusion:
The correct word is "sa".