The question has wrong options. There are 6 prime implicants ($W\bar{Z}$, $X\bar{Z}$, $W\bar{X}$, $WY$, $XY$ and $Y\bar{Z}$), and 2 essential prime implicants ($W\bar{X}$ and $Y\bar{Z}$).
The starred 1's in the below diagram indicate the minterms (i.e., 1's) that are responsible for making the enclosing prime implicant as essential prime implicant.