Which homophones correctly complete the sentence?

If they're locked out of their house, they should call me and I will let them in. (C).

They're, their, and there are all homophones or words that are pronounced the same. However they all have different meanings.

Therefore, we can replace they're with they are.

They are in possession of the house, so it is their house.

ynkofjfhhibn vf grr

i'm thinking it would be when compound subjects are joined by or, nor,either-or, neither-nor.