Y:
Please refer the following link:
http://www.igidr.ac.in/newspdf/advt2013.pdfIts clearly stated in this file that:
"The written test for both M.Sc. and M.Phil./Ph.D. will emphasize general aptitude,
analytical ability and mathematical skills at appropriate levels.
In addition M.Sc. students will
have the option of answering a section either on Economics or Mathematics at the
undergraduate level."
So, prepare for both Economics and Mathematics.