Alkynes exhibit 

A) Chain isomerism 

B) Position isomerism 

C) Functional group isomerism 

• The hydrocarbons containing carbon-carbon triple bonds are known as alkynes. They do not show cis-trans isomerism as the $\mathrm{HC}\equiv \mathrm{CH}$ has the same number of molecules attached to both the ends. 

Taking an example, 1-pentyne and 2-pentyne

${\mathrm{H}}_3\mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{C}\equiv \mathrm{CH}$

${\mathrm{H}}_3\mathrm{C}-{\mathrm{CH}}_2-\mathrm{C}\equiv \mathrm{C}-{\mathrm{CH}}_3$

These both are positional isomerism of pentyne. As alkynes are linear molecules they show chain isomerism as well as functional group and positional isomerism.

Therefore, option (D) is correct.