Which of the following acids is isomeric with phthalic acid?

The number of isomers of $C_5H_{12}$ is:

Identify the pairs of compounds that represent chain isomerism among the following:
$I. CH_3-CH_2-CH_2-CH_2-OH$ (Butan$-1-$ol)
$II. CH_3-CH_2-CH(OH)-CH_3$ (Butan$-2-$ol)
$III. (CH_3)_3C-OH$ ($2$-Methylpropan$-2-$ol)
$IV. (CH_3)_2CH-CH_2-OH$ ($2$-Methylpropan$-1-$ol)

Which of the following pairs of compounds are functional group isomers ?

$HCN$ and $HNC$ are known as .............

The number of structural isomers for $C_6H_{14}$ is