The correct options are
C (b) and (d) are functional isomers
D (a) and (c) are functional isomers.
Structural isomers which shows a difference in the functional groups present in the carbon chain are functional isomers.
(a) and (c) are not positional isomers the functional group is different in both. (a) and (c) are functional isomers.
In (a) and (c) , the functional groups are alcohol and ether respectively, and both have same molecular formula C4H10O.
In (b) and (d) , the functional groups present are phenol and alcohol respectively. Both have same molecular formula C8H10O. (b) and (d) are functional isomers