A single fixed pulley is used because it:
A. Has a mechanical advantage greater than \[1\] .
B. Has a velocity ratio less than \[1\] .
C. Gives \[100\% \] efficiency.
D. Helps to apply the effort in a convenient direction.

Hint: The single fixed pulley doesn't quite decrease effort, but helps to adjust the course of effort applied. The application of effort throughout the downward direction is much simpler, the single fixed pulley being commonly used.

Complete answer:
In the given question, we are given a single pulley and we are asked to find out which one of the above conditions satisfies the situation that we are given with.
To begin with, we know a calculation of the force enhancement obtained by the use of a device, mechanical unit or computer system, is a mechanical advantage. In order to produce a desired increase in the output energy, the unit retains the input power and merely exchanges powers against movement. The rule of the lever is the standard for this.

We also know that the velocity ratio of a pulley suggests that the pulley must come with \[1\,{\text{m}}\] of rope and so the load increases by \[1\,{\text{m}}\]. This implies that the pulley system's velocity ratio equals \[1\]. Even so, if the pulley is \[100\% \] effective and the speed ratio is \[1\], the mechanical advantage is \[1\] as well, which implies that the load or effort are the same.
There is no mechanical benefit of a single fixed pulley; it only helps to apply the action in the right direction.
Hence, it helps to apply the effort in a convenient direction.

Hence, option D is correct.

Note:
To lift the load while changing the direction of a lifting power, a single fixed pulley is being used. While there is no mechanical advantage with a single fixed pulley, the effort exerted is greater than that of the load, because you should not have to pull or drive the pulley up and down, it is preferable to use.