A single vector which can replace a system of vectors and has the same effect is called a:

A resultant is a single vector that has the same effect as a system of vectors acting simultaneously. It combines the magnitudes and directions of the individual vectors to produce a net effect. This concept is fundamental in physics and engineering, particularly in the study of forces, where multiple forces acting on an object can be represented as a single force for simplification.

By using the resultant, engineers and physicists can analyze systems more easily, as they can focus on one vector rather than multiple vectors. This is particularly useful in calculations involving equilibrium, motion, and other complex interactions of forces.

The other options, such as coplanar vector and concurrent vector, refer to specific geometric relationships between vectors but do not encompass the idea of representing multiple vectors with a single equivalent vector. A scalar is a quantity that has magnitude but no direction; it does not represent the vector nature that is required to replace a system of vectors.