Your sister will look after you.The given sentence is in active voice.
Right Answer is: C
The given sentence is in active voice. It is of simple future tense. Let us understand the structures for active/passive voices for such sentences.
Active: Subject + will/shall + verb (Ist form) + object…
Passive: Object+ will/shall + be + verb (IIIrd form) + by + subject…
So, with the help of the above structures, we can convert the sentence into passive voice: You will be looked after by your sister.
Hence, option C is the correct answer.