Which subject should I give special emphasis to, during my BE in civil engineering, if I intend to opt for a PGP REUIM at NICMAR?

Actually, all of the subjects have something that will be used in the future and that you must prepare for.

Concentrate on your fundamental civil engineering subjects, such as Concrete Technology and building materials. These topics will help you prepare for interviews if you are applying for specific positions in large real estate companies. 

For REUIM, you don't need to focus on any of the civil engineering-specific disciplines, in order to get admission. REUIM provides a better balance of electives, such as corporate real estate management, real estate negotiations, and asset management, among others. The rest of the numerous subjects are shared by all branches.