Assertion: Invar steel is used to prepare clock pendulum
Reason: The coefficient of linear expansion for A
(A) Both assertion and reason are correct and reason is the correct explanation for assertion
(B) Both assertion and reason are correct and reason is not the correct explanation for assertion
(C) Assertion is correct but reason is incorrect
(D) Both assertion and reason are incorrect

Hint:- Invar is a low expansion alloy. Thus this property will be useful in many appliances. We want to check whether the Invar is useful in a clock pendulum.

Complete step by step answer:
Invar is a low expansion alloy of Nickel and iron. In temperature between they can maintain constant dimensions. Hence they are used in the applications where we want high stability in dimensions. The major feature of Invar is that they have nearly zero coefficient of thermal expansion.
Thus the temperature differences would not affect the material. They don’t have any linear expansion. Thus the time will be accurate at every temperature. Thus they can be used in clock pendulums. Therefore throughout the seasons the clock pendulum will show correct time. The length of the pendulum will not change according to the changes in the external atmosphere.
Both assertion and reason are correct and reason is the correct explanation for assertion

The answer is option A.

Note: Other uses of the Invar includes, they are used in optical engineering due to high stability in dimensions. And also in measuring and positioning devices.