Assertion . K , Rb and Cs form superoxides .
Reason . The stability of the superoxides increases from K to Cs due to decrease in lattice energy .

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 assertion and reason question regarding the formation of superoxides by potassium (K), rubidium (Rb), and cesium (Cs), we will analyze both the assertion and the reason step by step.

### Step 1: Understanding the Assertion
The assertion states that K, Rb, and Cs form superoxides.

- **Superoxides** are compounds that contain the superoxide ion (O2^-).
- Alkali metals, such as K, Rb, and Cs, react with oxygen to form superoxides when they are heated in the presence of oxygen.

**Conclusion for Step 1**: The assertion is correct; K, Rb, and Cs indeed form superoxides (KO2, RbO2, CsO2).

### Step 2: Understanding the Reason
The reason states that the stability of the superoxides increases from K to Cs due to a decrease in lattice energy.

- **Lattice Energy** is the energy released when gaseous ions combine to form an ionic solid. It is influenced by the size of the ions involved.
- As we move down the group from K to Cs, the size of the cation increases. Larger cations lead to a decrease in lattice energy because the distance between the cation and anion increases, resulting in weaker electrostatic forces of attraction.

**Conclusion for Step 2**: The reason given is incorrect. The stability of superoxides actually increases from K to Cs due to an increase in lattice energy (not decrease), as the larger size of Cs allows for better stabilization of the superoxide ion.

### Final Conclusion
- The assertion is **true**: K, Rb, and Cs form superoxides.
- The reason is **false**: The stability of superoxides increases from K to Cs due to an **increase** in lattice energy, not a decrease.