Pointing to the photograph of Amit, Gagan said, 'His sister's mother's husband is my daughter's mother's husband's father.’ How is Amit related to Gagan?

According to the problem,

Amit's sister's mother's husband is Gagan's daughter's mother's husband's father

Sister's mother is mother. Daughter's mother is wife.

$$\Rightarrow$$ Amit's mother's husband is Gagan's wife's husband's father

Mother's husband is Father. Wife's husband is Gagan himself.

$$\Rightarrow$$ Amit's father is Gagan's father

$$\therefore\ $$Amit is the brother of Gagan

Hence, the correct answer is Option D