Final answer:
A Waterproof underpad is the equipment used by primary health care providers while applying moist dressing to a patient with pressure ulcers.
Step-by-step explanation:
When a primary health care provider is applying moist dressing to a patient with pressure ulcers, a Waterproof underpad (Option D) is typically used to protect the bed or surface under the patient. This helps prevent moisture from the dressing from damaging the bed or contaminating other surfaces. It is not only part of necessary sanitary protocols but also ensures comfort for the patient. Linen bags are for holding used linens, the Braden scale is a tool to assess the risk of developing pressure ulcers, and Montgomery ties are used for securing large dressings without the need for adhesive tapes.