Assertion (A) Aldehydes and ketones, both react with Tollen's reagent to form silver mirror.
Reason (R) Both, aldehydes and ketones contain a carbonyl group.

Explanation

To solve the question, we need to analyze the assertion (A) and the reason (R) provided.

### Step 1: Analyze the Assertion (A)
The assertion states that "Aldehydes and ketones, both react with Tollen's reagent to form silver mirror."

- Tollen's reagent (Ag(NH3)2^+) is known for its ability to oxidize aldehydes to carboxylic acids while reducing silver ions to metallic silver, which forms a silver mirror.
- However, ketones do not undergo this reaction because they are generally resistant to oxidation under mild conditions.

**Conclusion for Step 1**: The assertion is **false** because only aldehydes react with Tollen's reagent to form a silver mirror, while ketones do not.

### Step 2: Analyze the Reason (R)
The reason states that "Both aldehydes and ketones contain a carbonyl group."

- Both aldehydes (RCHO) and ketones (RC(=O)R') indeed contain a carbonyl group (C=O).
- The presence of the carbonyl group is a characteristic feature of both functional groups.

**Conclusion for Step 2**: The reason is **true** because both aldehydes and ketones have a carbonyl group.

### Final Conclusion
- Assertion (A) is **false**.
- Reason (R) is **true**.

Thus, the correct relationship is that the assertion is false while the reason is true.