Explanation

(b)
The sum of the oxidation states is always zero in neutral compound.
The oxidation state of $X,Y,$ and $Z$ are +2, +5 and $- 2\$respectively.
In $X_{2}YZ_{6}$
$2 \times 2 + 5 + 6( - 2) \neq 0$
In$\ XY_{2}Z_{6}$
$2 + 5 \times 2 + 6( - 2) = 0$
In $XY_{5}$
2+5$\times 5 \neq 0$
In $X_{3}YZ_{4}$
$3 \times 2 + 5 + 4( - 2) \neq 0$
Hence, the formula of the compound is $XY_{2}Z_{6}.$