$($a$)$ Write letters for any compounds that are constitutional isomers of each other.

$($b$)$ Write the letters for any compounds containing more than one $-$bond.

$($c$)$ Write the letter$($s$)$ for any compound$($s$)$ having DU $=2($same as HDI$).$

$($d$)$ Write the letter$($s$)$ for any compound$($s$)$ containing sp$-$hybridized atoms.

$($e$)$ Supply molecular formula for compound $C($convention $=C$&$H$ first, then alphabetic$).$