Concept:Equilibrium requires a zero resultant force. This is known as the first condition of equilibrium.Explanation:When the sum of all external forces acting on a body is zero, the body is said to be in translational equilibrium.This is written mathematically as ∑F=0.A zero net force means the body has no linear acceleration; it is either at rest or moving with constant velocity.Equilibrium also has a second condition, which involves the sum of torques: ∑τ=0.However, the statement "sum of all forces is zero" defines only the first condition of equilibrium.Therefore, the correct option is the First condition.Answer:A. First condition