Assertion: Cesium is the most electropositive element.
Reason: Cs has lowest electron affinity.

Options

A. Both A and B true and R is the correct explanation A
B. Both A and R are true but R is not a correct explanation of A
C. A is true but R is false
D. A is false but R is true

Answer

To solve the question regarding the assertion and reason about cesium, we will break it down into steps.

### Step 1: Analyze the Assertion
The assertion states that "Cesium is the most electropositive element."
- **Explanation**: Electropositivity refers to the tendency of an element to lose electrons and form positive ions. Cesium (Cs) is an alkali metal, and alkali metals are known for their high electropositivity. Among all alkali metals, cesium has the largest atomic radius and the lowest ionization energy, making it the most electropositive element.

### Step 2: Analyze the Reason
The reason states that "Cs has the lowest electron affinity."
- **Explanation**: Electron affinity is the energy change that occurs when an electron is added to a neutral atom. Cesium does indeed have a low electron affinity, meaning it does not favor gaining electrons. This is consistent with its electropositive nature, as elements that easily lose electrons (like cesium) typically do not gain them readily.

### Step 3: Determine the Relationship Between Assertion and Reason
- The assertion is true: Cesium is indeed the most electropositive element.
- The reason is also true: Cs has the lowest electron affinity.
- However, the reason does not correctly explain the assertion. The assertion is better explained by the low ionization energy of cesium rather than its electron affinity.

### Conclusion
Based on the analysis:
- Both the assertion and reason are true, but the reason is not the correct explanation for the assertion. Therefore, the correct answer is that both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

### Final Answer
B: Both assertion and reason are true, but the reason is not the correct explanation of the assertion.