Answer: seiner
Step-by-step explanation:
Hey, here, "seiner Mutter" ist a direct object complement (I hope the translation is good), and it should be in AKKUSATIV but here it's the verb "helfen" + DATIV, so you have to choose "seiner", since it's in DATIV and feminine ( sorry if the translation is bad again, it is die Mutter).
seine : nominativ f.
seinen : nominativ m.
seinem: dativ m. or neutral