The four basic lifting principles that govern a crane's mobility and safety during lifting operations are leverage, structural integrity, stability, and

And center of gravity.
When it comes to cranes and their stability and mobility, center of gravity actually depends on the weight of that object being moved by the crane. It will be located based on the heaviest object being carried. Center of gravity is actually the point within that particular object where weight is evenly distributed around it. 

Ibelieve your answer would be answer choice a