Final answer:
The minimum number of non-collinear forces required for a body to be in equilibrium is three. These forces must vectorially add up to zero to satisfy the first condition for equilibrium according to Newton's second law.
Step-by-step explanation:
If a body is in equilibrium under a set of non-collinear forces, then the minimum number of forces acting on it must be three (option c). This is because these forces, when added as vectors, must counterbalance each other to maintain a state of equilibrium. In the case of three forces, they must follow the principle of a closed triangle, where the vector sum of the forces equals zero, thereby satisfying the first condition for equilibrium according to Newton's second law.
For example, if we consider forces acting in a two-dimensional plane, we could have a force A acting to the right, a force B acting to the left, and a force C acting downward. For the object to be in equilibrium or to move straight down without accelerating sideways, the horizontal components of A and B must cancel, while the vertical component must be balanced by another force, or the weight of the object if force C is upward. In the context of free-body diagrams, this would be depicted by vector arrows representing forces, with their direction and magnitude corresponding to the forces acting on the body.