Assertion. C-H bond in ethyne is shorter than C-H bonds in ethene.
Reason. Carbon atom in ethene is sp hybridised while it is `sp^3` in ethyne.

Options

A. If both assertion and reason are true, and reason is the true explanation of the assertion.
B. If both assertion and reason are true, but reason is not the true explanation of the assertion.
C. If assertion is true, but reason is false.
D. If both assertion and reason are false

Explanation

To solve the question, we need to analyze the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that the C-H bond in ethyne (C2H2) is shorter than the C-H bond in ethene (C2H4).

- Ethyne has a triple bond between the carbon atoms (C≡C), and each carbon is bonded to one hydrogen atom.
- Ethene has a double bond between the carbon atoms (C=C), and each carbon is bonded to two hydrogen atoms.

### Step 2: Compare the C-H Bonds
The bond length is influenced by the type of hybridization and the bond order:
- In ethyne, the carbon atoms are sp hybridized, which means they have a higher s-character (50% s and 50% p). This results in stronger and shorter bonds.
- In ethene, the carbon atoms are sp² hybridized (33% s and 67% p), which results in longer C-H bonds compared to ethyne.

Thus, the assertion is correct: the C-H bond in ethyne is indeed shorter than in ethene.

### Step 3: Understand the Reason
The reason states that the carbon atom in ethene is sp hybridized while it is sp³ in ethyne.

- This statement is incorrect. In ethene, the carbon atoms are sp² hybridized, not sp hybridized.
- In ethyne, the carbon atoms are sp hybridized.

### Step 4: Conclusion
- The assertion is true: C-H bond in ethyne is shorter than in ethene.
- The reason is false: the hybridization of carbon in ethene is sp², not sp.

### Final Answer
The assertion is true, but the reason is false.