Seven people, A, B, L, M, N, S and T, are sitting in a row, facing north(but not necessarily in the same order). Only three people sit to the left of T. Only B sits to the right of A. Only three people sit between A and S. L sits at some place to the left of N but at some place to the right of M. How many people sit between M and N?

Seven people: A, B, L, M, N, S, T are sitting in a row, facing North.
Only three people sit to the left of T.
Only B sits to the right of A.
Only three people sit between A and S.
L sits at some place to the left of N but at some place to the right of M.

According to the given information, Seating arrangements will be:

As per the arrangements, there are three people (S, L and T) sit between M and N.

Thus, the correct option is (a) Three.