According to me,

Ans = 4.

Explaination:-

If units digit of x^2 = 6. It means x's units digit has to be 6.

For Numbers having units digit as 6 , their squares units digit will be 6. (becos 6 ^ 2 = 36)

Similarly if units digit of y^2 = 9, means y's units digit is either 3 or 7, since square of both will give a 9 in units place.

Since x and y are consecutive y's units digit should be 7.

Hence z's units digit should be 8.

square of a number whose units digit is 8, should have its units digit as 4, since 8 ^2 = 64

Whats the official Ans?

punit.kaur.mba,
you are correct. I got it now. Thanks