A prime contractor is named an additional insured on a subcontractor's CGL policy. The practical benefit to the prime is that:

Explanation

Additional insured status extends the subcontractor's coverage to the prime for liability arising out of the subcontractor's operations, so the sub's insurer may owe the prime a defense and indemnity for those claims. (b) is the dangerous error: the coverage reaches only the sub's work, is capped by the sub's limits, and is subject to the sub's exclusions, so the prime's own exposures remain uninsured. (c) confuses being insured with owning the policy — the subcontractor buys it, controls it, and can let it lapse, which is why the prime asks for the endorsement and for notice of cancellation. (a) has nothing to do with the endorsement.