Given $f(A,B,C,D)={\displaystyle \underset{?}{\overset{?}{}}}(8,9,11,12,15)$ The Essential Prime Implicants are

a$.\frac{A}{b}ar\frac{C}{b}ar(D),$ACD only

b$.\frac{A}{b}ar\frac{C}{b}ar(D),\frac{A}{b}ar\frac{B}{b}ar(C),A\frac{C}{b}ar(D)$

c$.\frac{A}{b}ar(B)D,\frac{A}{b}ar\frac{B}{b}ar(C)$ only

d$.\frac{A}{b}ar\frac{C}{b}ar(D),$ACD,$\frac{A}{b}ar(B)D$