What is the minimum number of carbon atoms required to exhibit position isomerism in an organic compound?

Which of the following pairs are not isomeric compounds?

Which of the following pairs of compounds are positional isomers?

For which of the following factors can the structural isomers $C_2H_5OH$ and $CH_3OCH_3$ be assumed to have the same value? (Assuming ideal behavior)

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

Which molecular formula does not exhibit chain isomerism?