The correct option is C Both X and Y
Section 141(1) and Section 141(2) of Companies Act, 2013 contains provision as regards to qualifications of auditors. A person shall not be qualified for appointment as auditor of a company unless he is a chartered accountant within the meaning of the Chartered Accountants Act, 1949.
In this XYZ is a chartered accountancy firm which implies that all the partners are chartered accountants. Thus if Y has been appointed as partner of XYZ then he is also CA. Thus all partners of XYZ firm can be appointed as auditors. So, Both X and Y will be the auditors.