Your solution is giving WA because you are passing 2nbin to your function. 600000 in binary is close to 1.1E19 and range of ULL is 1.8E19. So 2nbin will go out of range. What you can do is make two calls to the modular exponent function, first to calculate 2^nbin and then (result of first call)^2.
It is showing time limit exceeded what should I do to optimize the program?