What are isomers called that have different structures and exist in equilibrium with each other?

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

$A$ compound whose molecules are superimposable on their mirror images even though they contain an asymmetric carbon atom is called:

The number of possible isomers for ${C_7}{H_8}O$ is ......

For which of the following parameters would the structural isomers $C_2H_5OH$ and $CH_3OCH_3$ be expected to have the same values? (Assume ideal behavior)

Minimum number of carbon atoms required for a compound to show positional isomerism?