Final answer:
A curette is the correct instrument for removing subgingival plaque, as it's designed to work on root surfaces below the gumline. Option a.
Step-by-step explanation:
The instrument used for removing subgingival plaque is a curette option a. A curette is specially designed to adapt to the curvatures of the root surfaces and allows for careful removal of plaque and calculus from below the gumline, which is essential for treating periodontal disease. In contrast, scalers are more often used for super gingival plaque removal, elevators are used in oral surgery to lift teeth or bone, and calculus forceps are designed to remove larger pieces of calculus but not focused on fine subgingival plaque removal.