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

Which of the following pairs of compounds are functional isomers?

The following are the structures of four isomers of hexane. Among the names given below,which correctly identifies the fifth isomer?
$CH_3CH_2CH_2CH_2CH_2CH_3$
$(CH_3)_3CCH_2CH_3$
$(CH_3)_2CHCH_2CH_2CH_3$
$(CH_3)_2CHCH(CH_3)_2$

Which of the following is a pair of metamers?

Which molecular formula does not exhibit chain isomerism?

Among the following,the most stable compound is: