The correct answer is D. A commercial umbrella policy provides an additional layer of liability insurance above specified underlying policies, such as Commercial General Liability, Business Auto Liability, or Employers Liability. For a loss covered by both the underlying policy and umbrella, the umbrella generally begins responding after the applicable underlying limits have been exhausted by covered loss, subject to the umbrella's terms, limits, exclusions, and attachment requirements.

Travelers describes umbrella coverage as providing additional limits for types of losses covered by primary policies, while excess liability functions as another layer over primary protection. The Series 17-70 outline specifically tests commercial umbrella coverage, underlying limits, excess coverage, stand-alone coverage, and follow-form concepts.

Option A is incorrect because an umbrella ordinarily does not automatically drop down merely because an underlying insurer becomes insolvent; insolvency treatment depends on the umbrella's exact wording. Option B refers to a personal-auto exposure rather than the commercial liability structure asked about. Option C is incorrect because a loss still within the retained underlying limit is principally the responsibility of the underlying insurer or insured retention.

Thus, once the required underlying policy limits have been exhausted by covered claims, the umbrella layer can attach.

Therefore, D is correct.