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.
Anil introduces Rohit as the son of the only brother of his father's wife.
How is Rohit related to Anil ?
The relations may be analysed as follows :
Father's wife -- Mother;
Mother's brother -- Uncle;
Uncle's son -- Cousin.
So, Rohit is Anil's Cousin.
Hence, the answer is (a).