For example, I am my father's son. If I look at a picture of myself, I would say "that (this) man's father (me) is my father's son" (which I am).
What, you are the father of yourself?
If you point at the picture of yourself and say "this man's father", that would be your father, where "this man" = you.
If you point at the picture of your son and say "this man's father", that would be you, where "this man" = your son.
Put another way, since you are viewing a picture of yourself, then "this man" = you/your = my. So, "your father is your father's son". It doesn't make sense.
FWIW, I believe the way it is worded, the answer is the viewer's son. However, I think they meant to say, "This man is my father's son".
This poster said it best, and I saw it in a different light:
This man = SON_A
This man's father = DAD_A
My father = DAD_B
My father's son = SON_B
The quote states that DAD_A = SON_B
Which makes SON_B father of SON_A.
So the answer is "his son", not one of those listed!
Let's say the family tree goes like this (males only):
Fred (grandfather) -> Bob (son of Fred) -> Reggie (Bob's son, Fred's grandson)
If the viewer, Bob, is looking at a photo of his son, Reggie, the quote then turns to:
"Reggie's father is Fred's son"
where:
Reggie = "this man" (man in photo)
Fred = "my father" (the father of the viewer, Bob)
If the viewer, Bob, was looking at himself, it would translate to (and doesn't make any sense):
"Fred is Fred's son"
where:
Bob = "this man" (man in photo and viewer)
Fred = "my father" (the father of the viewer, Bob)