The correct answer is: D. The absolute roughness increases with time.
The absolute roughness of a pipe is a measure of the small irregularities on the surface of the pipe. These irregularities can cause friction between the fluid flowing through the pipe and the pipe wall. As the pipe ages, these irregularities can become larger, which increases the absolute roughness of the pipe. This increase in absolute roughness can lead to an increase in friction and a decrease in the flow rate of the fluid.
Option A is incorrect because the absolute roughness of a pipe does not decrease with time. In fact, it increases with time.
Option B is incorrect because a pipe does not become smooth after using for long time. In fact, the opposite is true. The pipe becomes rougher with time.
Option C is incorrect because the friction factor does not decrease with time. In fact, it increases with time. This is because the increase in absolute roughness causes an increase in friction.