Fill in the blank with the correct singular possessive adjective: c'est tante au téléphone.

Hi !

fill in the blank with the correct singular possessive adjective =

we don't know which aunt is it, isn't ?? could be mine ma), your(ta), his, her  (sa)

I think it's a trap, a pitfall to see if you use his or her in French correctly (see below what I've trying to explain : his/her aunt = both sa tante (read below)
that's why I would answer :

C'est SA tante au téléphone  = it's her/his aunt on the phone

possessive French  adjective =

My = mon : sing. masc// ma : sing. fem// mes : plural
Your = ton : sing. masc //ta : sing. Fem // tes : plural (for a "tutoiement")
His = son (agrees with the following word) as in English agrees with the              owner
her = sa   ( same)
its = ses    (when the following word is plural)
our = notre : sing. // nos :  plural
your= votre : sing.  // vous : plural (same for 2nd person plural and "vous de             politesse)
their = leur/s

now there are differences concerning with what the possessive  adjective has to agree
It agrees in gender and number with the following word as in English it agrees with the owner
for example you'll say : his  aunt if it's the aunt of a man
In French as aunt is feminine, it will always be = sa tante

That's why I'm pretty sure the answer is sa tante ; nothing indicates you if it's mine, your??? !!!

hope I'm right for the" trap" and that my attempt of explanations are helpul
(sorry for my English, I'm French I do my best !)