The isomer of propyne is:A. AlleneB. PropeneC. CyclopropaneD. Propane

The question asks for the isomer of **propyne**. Let's break it down:

**Propyne (C₃H₄)** is an alkyne, meaning it has a triple bond between two carbon atoms. Its structure is:

\[ H-C≡C-CH₃ \]

Now, let's analyze the options for the isomer:

- **Allene (C₃H₄)**: Allene is also a hydrocarbon with the formula **C₃H₄**, but it has a different bonding structure. Instead of a triple bond, it has two double bonds: one between the first and second carbons, and another between the second and third carbons. The structure of allene is:

\[ H₂C=C=CH₂ \]

Allene has the same molecular formula as propyne (C₃H₄), but a different arrangement of atoms, making it an isomer of propyne.

- **Propene (C₃H₆)**: Propene is an alkene with the formula **C₃H₆** and has a double bond between the first and second carbons:

\[ CH₃-CH=CH₂ \]

Since the molecular formula is different (C₃H₆ instead of C₃H₄), propene is not an isomer of propyne.

- **Cyclopropane (C₃H₆)**: Cyclopropane is a cyclic hydrocarbon with the formula **C₃H₆**, where three carbon atoms form a ring. Again, its molecular formula is different from that of propyne, so it is not an isomer of propyne.

- **Propane (C₃H₈)**: Propane is an alkane with the formula **C₃H₈**, which means it has only single bonds between the carbon atoms. The molecular formula is different from propyne, so propane is not an isomer.

### Conclusion:
The correct answer is **A. Allene**, because it has the same molecular formula (C₃H₄) but a different structure compared to propyne.