Which of the following is not an isomer of $but-1-yne$?

$n$-Butyl alcohol and isobutyl alcohol are ............... .

Which of the following is not a pair of functional isomers?

The total number of structural isomers of $C_3H_6O$ is:

The number of metamers possible for $C_{4}H_{10}O$ is

Which of the following methods of structural formula representation conventionally uses a point for the front carbon and a circle around it for the rear carbon?