Assertion: Among hydroxides of alkali metals, LiOH is the weakest base.
Reason: Among alkali metals, lithium has the highest ionisation energy .

Options

A. If both assertion and reason are true and the reason is the correct explanation of assertion
B. If both assertion and reason are true and the reason is not the correct explanation of assertion
C. If assertion is false but reason is true
D. If assertion is true but the reason is false

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that among the hydroxides of alkali metals, lithium hydroxide (LiOH) is the weakest base. To evaluate this, we need to consider the properties of alkali metal hydroxides.

2. **Properties of Alkali Metal Hydroxides**: Alkali metal hydroxides generally increase in basicity as we move down the group from lithium to cesium. This is because the solubility and dissociation of these hydroxides in water increase, leading to stronger basic solutions.

3. **Evaluating Lithium Hydroxide**: Lithium hydroxide is indeed less soluble compared to other alkali metal hydroxides such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). Its lower solubility results in a lower concentration of hydroxide ions (OH⁻) in solution, making it a weaker base.

4. **Understanding the Reason**: The reason states that lithium has the highest ionization energy among alkali metals. This is true; lithium's ionization energy is higher than that of other alkali metals due to its smaller atomic size and greater effective nuclear charge.

5. **Linking Reason to Assertion**: While both the assertion and reason are true, the reason does not directly explain why LiOH is the weakest base. The weakness of LiOH as a base is more closely related to its high hydration energy and lower solubility rather than solely its high ionization energy.

6. **Conclusion**: Therefore, both the assertion and reason are true, but the reason is not the correct explanation for the assertion. The correct answer is that both statements are true, but the reason does not explain the assertion.

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