Applied Mechanics and Graphic Statics
The necessary condition of equilibrium of a body is:

Algebraic sum of the moments of the forces about a point must be zero
Algebraic sum of vertical components of all the forces must be zero
All (A), (B) and (C)
Algebraic sum of horizontal components of all the forces must be zero

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Applied Mechanics and Graphic Statics
If a set of given forces are such that their free vectors build a closed polygon, then

The resultant force is zero but resultant couple is not zero
The resultant force is zero but resultant couple may not be zero
The resultant force and resultant couple both may not be zero
The resultant force and resultant couple are always zero

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