Methane ,alkenes& alkynes do not give conformational isomers why?

Methane doesn’t have any isomers.

Methane only has 1 carbon atom so there is no chain of carbon atoms to make structural isomers.

There just isn’t any way to rearrange the molecule differently.

Conformational isomers are only possible when there is a free rotation around C-C bond. In case of alkenes and alkynes there is no free rotation around the Carbon atoms due to pi bond. The pi bond does not allow free roataion and has restricted roatation. Thus Confromational isomers are possible in alkanes and its derivatives.