A $3$ line to $8$ line Decoder is used to implement a $3$-variable Boolean function as shown in figure.
The simplified form of output $Y$ is.
HERE its written like this(Z is MSB)
NOW TAKE 0's outside and it will be NAND GATE now it says when enteries are either (1,3,5,6) we get 1
so fill 1 at those places and it will turn out to be Z'Y'X+Z'YX+ZY'X+ZYX' THAT MATCHES WITH c
JUST MAKE THE OPTION C IN CANONICAL FORM