Why might you want to use a single fixed pulley?
to raise or lower heavy objects
to raise or lower lightweight objects
to reduce the force needed to raise objects
to reduce the weight of lifted objects

A single fixed pulley can be used to raise or lower lightweight objects.

Option b

Explanation:

A pulley is a simple machine tool which is used to make lifting or lowering tasks easy.  A single fixed pulley is a system involving only one pulley fixed on a constant rigid support with a rope wrapped around the wheel. Such a system can be used only to change the direction of  applied force in raising or lowering small, lightweight objects which need minimal work force.

A single fixed pulley system helps only in redirecting the applied force direction by using a rope and wheel assembly. The work done in such a case remains the same and hence it is not preferred to use it in lifting heavy objects. Neither is the required force reduced in case of a single fixed pulley system. A movable pulley helps in achieving (A) and (C).

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c) both a and b.