this uncle

[azz]

I and a friend of mine are looking at a photo of a group of people, three of whom are our friend Janet's uncles.
I say: Janet's uncle looks unwell.
My friend says: No, he doesn't.
I realize that my friend is not looking at the uncle I am talking about..

I point to the man I am talking about and say

a. I am talking about this one of Janet's uncles.
b. I am talking about this uncle of Janet's.
c. I am talking about this uncle of Janet.

Which of the sentences a-c are grammatically correct and can be used in this context?

Many thanks.

[Alan]

b only