Work from the inside out. 'Rohan's mother's father-in-law' means the father of Rohan's mother's husband — that husband is Rohan's father, so this person is Rohan's father's father, i.e. Rohan's grandfather. The 'only daughter' of this grandfather is Rohan's father's only sister, i.e. Rohan's aunt (bua/father's sister). So the lady in question is Father's Sister, option (3).