Assertion : In alkanes, alkenes and alkynes the valency of carbon is always four.
Reaction : All hydrocarbons except alkanes contain double bonds.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of 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 reaction provided.

### Step 1: Understand the Assertion
The assertion states that "In alkanes, alkenes, and alkynes, the valency of carbon is always four."

- Carbon has four valence electrons and can form four bonds to satisfy its tetravalency. This is true for all types of hydrocarbons:
- **Alkanes** (single bonds): Example - Methane (CH₄) where carbon forms four single bonds with hydrogen.
- **Alkenes** (double bonds): Example - Ethene (C₂H₄) where each carbon is bonded to two hydrogens and one carbon with a double bond.
- **Alkynes** (triple bonds): Example - Ethyne (C₂H₂) where each carbon is bonded to one hydrogen and one carbon with a triple bond.

### Step 2: Understand the Reaction
The reaction states that "All hydrocarbons except alkanes contain double bonds."

- This statement is incorrect. While alkenes do contain double bonds, alkanes do not. Alkanes consist solely of single bonds. Additionally, alkynes contain triple bonds, not double bonds.

### Step 3: Conclusion
Based on the analysis:
- The assertion is **true** because carbon always has a valency of four in all hydrocarbons.
- The reaction is **false** because not all hydrocarbons except alkanes contain double bonds; alkynes contain triple bonds.

Thus, the correct answer is that the assertion is true, but the reason is false.

### Final Answer:
The assertion is true, and the reaction is false.

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