Ganesh was taking a walk with his mother’s brother’s father’s grand-daughter. Who was he walking with?

The correct option is Cousin.

We can solve this blood relation riddle by breaking down the relation step-by-step from Ganesh's perspective:

1. Mother's brother: Ganesh's mother's brother is his maternal uncle.
2. Mother's brother's father: The father of Ganesh's maternal uncle is Ganesh's maternal grandfather.
3. Mother's brother's father's grand-daughter: The granddaughter of Ganesh's maternal grandfather would be the daughter of either Ganesh's mother (making her Ganesh's sister) or Ganesh's maternal uncle (making her Ganesh's cousin).

Since "Sister" is not one of the provided choices, the granddaughter must be his maternal uncle's daughter, which makes her his cousin.
Therefore, Ganesh was walking with his cousin.