A logical function is given as F(A, B, C, D) = Σm (0, 4, 5, 10, 11, 13, 15). The number of Essential Prime Implicants in the given function will be _________.

Essential prime implicant is basically a prime implicant , but it can cover outputs of a function which no other combination of prime implicants can cover.
1 at 0000 and at 1010 has only one possible way to be grouped , but others have two possibilities.
Thus there are only 2 essential prime implicants , which shall exist in all possible groupings of 1's at 0000 and at 1010