Assertion : Benzaldehyde is less reactive than propanal towards nucleophilic addition reactions.
Reason : Benzaldehyde is less sterically hindred.

Answer

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

**Assertion**: Benzaldehyde is less reactive than propanal towards nucleophilic addition reactions.

**Reason**: Benzaldehyde is less sterically hindered.

### Step-by-Step Solution:

1. **Understanding Reactivity in Nucleophilic Addition**:
- Nucleophilic addition reactions involve the attack of a nucleophile (a species with a negative charge) on a carbonyl carbon (C=O) that is electrophilic (partially positive).
- The reactivity of carbonyl compounds in nucleophilic addition is influenced by the positive charge density on the carbonyl carbon.

2. **Analyzing Benzaldehyde and Propanal**:
- **Benzaldehyde** (C₆H₅CHO) has a carbonyl group attached to a benzene ring.
- **Propanal** (C₃H₇CHO) has a carbonyl group attached to an alkyl group (propyl).
- The benzene ring in benzaldehyde can stabilize the positive charge through resonance (the +R effect), which decreases the electrophilicity of the carbonyl carbon, making it less reactive towards nucleophiles.

3. **Comparing Steric Hindrance**:
- Steric hindrance refers to the crowding around the reactive site, which can impede the approach of nucleophiles.
- Benzaldehyde is not sterically hindered in the same way as propanal. However, the presence of the bulky benzene ring can create some steric hindrance compared to the smaller propyl group in propanal.
- Thus, while benzaldehyde is less sterically hindered than some other compounds, it is more hindered than propanal due to the presence of the benzene ring.

4. **Conclusion**:
- The assertion that benzaldehyde is less reactive than propanal is **true** because the resonance stabilization in benzaldehyde reduces its electrophilicity.
- The reason provided, that benzaldehyde is less sterically hindered, is **false** because it is actually more sterically hindered compared to propanal due to the presence of the benzene ring.

### Final Answer:
- The assertion is true, but the reason is false. Therefore, the correct option is **C**: Assertion is true, but reason is false.