Concept:
When several forces act on a body, their combined effect can often be represented by one equivalent force.
Step 1: The single force which produces the same effect as several forces is called the resultant force.
Step 2: The process of finding this resultant force is called composition of forces.
Step 3: Resolution of forces means splitting one force into components.
Step 4: Equilibrium means the resultant force is zero.
Step 5: Therefore, the correct answer is composition of forces.
\[
\boxed{\text{Composition of forces}}
\]