Assertion: Machine parts are jammed in winter.
Reason: The viscosity of the lubricants used in machine parts increase due to low temperature.
A) If ASSERTION and REASON are true and REASON is the correct explanation for ASSERTION.
B) If ASSERTION and REASON are individually true but REASON is not the correct explanation for ASSERTION.
C) If ASSERTION is true but REASON is false.
D) If Both ASSERTION and REASON are false.
E) If REASON is true but the ASSERTION is false.

Hint: To solve this question we should try to understand the effect of temperature on a lubricant and also we must know the use of a lubricant in a machine. If we figure out the relation among these three things we will easily reach to the conclusion.

Complete step by step answer:
We all know when two objects come in contact with each other, friction is produced. In the case of machines, friction is produced due to continuous movements of its parts. The friction so produced, causes wear and tear of the machine parts. This wear and tear can result in an untimely breakdown of the machine. To reduce this we use lubricants. Lubricants are fluids with low viscosity which reduces the friction among the machine parts and thus help to increase the machine longevity.
The value for viscosity of any fluid can be given by the given formulae-
$\eta = \dfrac{{2g{a^2}(\Delta \rho )}}{{9v}}$
Where $\eta $ is the viscosity of the fluid, $g$ is acceleration due to gravity, $a$ is the radius of the sphere which was used to measure the viscosity of the fluid, $\Delta \rho $ is the difference in density of the fluid and the sphere which was used to measure the viscosity of the fluid and and $v$ is the velocity of the sphere in the fluid.

In winters, the temperature decreases. The decrease in temperature causes increase in density having more effect on fluids then solids. Hence the density of the fluid increases. This causes an increase in $\Delta \rho $, which increases $\eta$ and thus the ease of movement between machine parts decreases causing jamming.

So the answer is the REASON is the correct explanation of the ASSERTION. So (A) is the answer.

Note: We can also use the concept of velocity. When temperature increases, the kinetic energy of the particles also increases and thus easy flow of the particles but when temperature decreases the movement becomes slow due to less kinetic energy.