In decipher jumbled up description type of questions, a round-about description is given in the form of certain small relationships and direct relationship between the persons concerned is to be deciphered.
X introduces Y saying, "He is the husband of the grand daughter of the father of my father."
How is Y related to X ?
The relations may be analysed as follows :
Father's father -- Grandfather;
Grandfather's Grand daughter -- Sister;
Sister's husband -- Brother-in-law.
So, Y is X's brother-in-law.
Hence, the answer is (c).