The boolean function for a combinational circuit with four inputs is represented by the following Karnaugh map.
Which of the product terms given below is an essential prime implicant of the function?
Only the top leftmost and bottom rightmost $1s$ have no alternate groupings. So, they form the essential prime implicants.
Answer is D. $Q'S'$
Essential prime implicants which $r$ grouped only by only one method or way,
So, in above question cornor's ones r grouped by only one method,
d ) will be the answer.