Which of the following is position isomer of?
Compounds having the same structure of the carbon chain but differ only in the position of the multiple (double or triple) bonds or the functional group are called positional isomers and the phenomenon is called positional isomerism.
Now, option a and b cannot be isomers because the molecular formula is not same.
But take a look at option C
There is a difference in the position of the double bond.
Hence (C) is the correct answer.