Number of prime implicants
By the definition:


  A prime implicant of a function is an implicant that cannot be covered
  by a more general, (more reduced - meaning with fewer literals)
  implicant.


it is a prime implicant as it cannot be covered by some other (single) implicant.

But it is not an essential prime implicant defined as:


  Essential prime implicants (aka core prime implicants) are prime
  implicants that cover an output of the function that no combination of
  other prime implicants is able to cover.


because it is fully covered by two other implicants.