A

Assertion Analysis:
The statement that while measuring the thermal conductivity of a liquid, the upper layer is kept hot and the lower layer is kept cold is correct. This setup helps to create a temperature gradient necessary for measuring thermal conductivity accurately.

Reason Analysis:
The reason given is that this setup avoids heating of the liquid by convection. This is also true because maintaining a temperature difference between the upper and lower layers minimizes convection currents, ensuring that heat transfer occurs primarily through conduction, which is essential for measuring thermal conductivity accurately.

Conclusion:
Both the assertion and the reason are true, and the reason correctly explains the assertion.
Therefore, the correct answer is A.