Which of the following acids is isomeric with phthalic acid?

An isomer of ethanol is

The number of possible isomers for the molecular formula $C_2H_3Cl_3$ is...........

Which of the following will have a meso isomer?

For the following four structures $I$ to $IV$,which statement is correct?
$(I)$ $C_2H_5-CH(CH_3)-C_3H_7$
$(II)$ $C_2H_5-CO-CH(CH_3)-C_2H_5$
$(III)$ $CH_3^+$ (Methyl carbocation)
$(IV)$ $C_2H_5-CH(CH_3)-C_2H_5$

Metamerism is shown by $:-$