A distance-time curve whose slope decreases with time represents:

Deceleration follows directly from the definitions and relationships used in kinematics. The question tests whether the learner can distinguish the required quantity from related ideas and use the stated information without adding unsupported assumptions. The alternatives Acceleration, Uniform speed, and Rest do not satisfy the governing definition, graph interpretation, direction convention, or numerical relationship in this case. In examinations, identify the known quantities, keep SI units consistent, select a positive direction where needed, and apply the appropriate definition or equation. This also checks that the answer has a sensible magnitude and unit.