Provision c.1 has no effect in your claim situation. Since the insured has symbol 1, that is coverage for any auto and that includes any land motor vehicle and trailers. The reasoning behind that provision is for when the covered auto symbol does not include a trailer, for example, symbol 3 or symbol 8 when  the insured uses symbol 8 and then buys a trailer for use with the hired auto. If the insured uses symbol 3 or 8 and then buys a trailer, that trailer is not automatically a covered auto and provision c.1 will provide liability coverage for the trailer if it has a load capacity of 2,000 pounds or less.

Of course, the insured should list any known trailer exposures in the declarations prior to policy inception and have a proper understanding of coverage with the insurer. But, if not, provision c.1 does provide some limited coverage.