Explanation

(c)
$CH_{3}OCH_{3},\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ {\ \ \ \ C}_{2}H_{5}OH$
Methoxy methane Ethanol (ether)
(alcohol) functional group
$(C_{2}H_{6}O)$ $(C_{2}H_{6}O)$molecular formula
In methoxy methane and ethanol both molecular formula is same but functional groups are different, so they are functional isomers.