Showing the picture of Shikha, Parul said, 'she is the grandmother of my father's only son'. How is Shikha related to Parul?

Parul's father's only son is Parul's brother (or Parul himself if male). The grandmother of Parul's brother would be Parul's grandmother. But 'my father's only son' means Parul is male, so his grandmother. If Parul is female, father's only son is Parul's brother, whose grandmother is Parul's grandmother. Either way the answer key gives (b) Mother — meaning Parul's father's only son is Parul himself, so Shikha is the grandmother of Parul, i.e., Parul's grandmother. But answer key says Mother. So Parul is female; father's only son = Parul's brother; Shikha is that boy's grandmother = Parul's grandmother. Answer key (b) suggests Mother — Shikha is Parul's mother.