One of the conditions that a force system is in equilibrium is:

A force system is considered to be in equilibrium when it satisfies certain conditions that ensure there is no net force or net moment acting on the system. One primary condition for a system to be in static equilibrium is that the sum of the upward forces must equal the sum of the downward forces. This balance prevents any movement in the vertical direction, which indicates that the forces are balanced.

When upward forces are equal to downward forces, it implies that the system is stable and not accelerating in any direction vertically. This condition is fundamental in structures and mechanical systems where maintaining stability under various loads is crucial.

In contrast, the other options present scenarios that do not represent equilibrium. The requirement that the resultant of forces may cause rotation indicates an unbalanced system, while the notion that all forces must act in one direction negates the possibility of forces opposing one another, which is essential for equilibrium in a multi-directional force system. Lastly, if the resultant of the forces causes an acceleration, it signifies that the system is not at rest and is experiencing motion, which again contradicts the principles of equilibrium.