A subcontract requires 'primary and noncontributory' coverage when the sub names the GC as additional insured. That means the sub's policy should:

Explanation

Primary and noncontributory means the subcontractor's policy answers first for claims arising out of the subcontractor's work, and does not ask the additional insured's own insurer to share the loss — which is what makes the risk transfer real rather than nominal. (c) is the excess or umbrella position, the exact inverse of what the requirement buys. (a) describes where the general contractor stands with no additional insured endorsement at all. (d) is workers' compensation, a coverage no liability endorsement supplies.