"I am with my mother."
Translation:मैं अपनी माँ के साथ हूँ।
We use the plural to show respect and अपनी is used for both singular and plural fem nouns.
अपने/अपनी mainly denotes possession or relation. With female subjects, singular or plural, अपनी has to be used.
अपने पिता - My (own) father
अपनी माँ - My (own) mother
अपना घर - My (own) house
अपनी दुकान - My(own) shop
I got the answer correct but am curious, why के सथ and not की सथ? Does सथ count as a post position as well?
"के साथ" works as one unit. I don't know if linguists (for example) would technically say साथ is a postposition according to some strict definition, but for all intents and purposes of learning Hindi, everyone treats it as such.
के (invariably, nothing to do with gender or number) appears in some two-part postpositions. It's a bit like saying "out of the water" in English, where "out of" is two words that work as a unit as the preposition. You couldn't say, "He jumped of the water." You could say: 1) "He jumped out the water" and 2) "He jumped out of the water." I feel that #1 is more colloquial whereas #2 is how I would express it in clear writing. Similarly, the के can be left off many times but it tends to be more clear to include it.
Whenever the subject of the clause is the one that is the owner of the object then you use अपना, अपनी, अपने. They literally mean "own" as in "my own" or "your own" or "his/her own". So तुम अपना सेब खाते हो। "you eat your (own) apple". मैं अपने बेटे के साथ घर जाता हूँ। "I go home with my (own) sons". वह अपनी किताब से पढ़ती है। "She reads from her (own) book". I hope that helps.