Final answer:
A warm front moves warm air over cold air with a gentle slope and typically moves in a northeastward or eastward direction.
Step-by-step explanation:
A warm front is defined as a boundary in meteorology where a mass of warm air advances and replaces a body of colder air. This front has a gently sloping surface, which allows the warm air to slowly rise over the cold air mass. Typically, in the mid-latitudes, a warm front moves from the southwest to the northeast, which means option (a) Advances warm air; gently sloping; westward is incorrect since warm fronts do not generally move westward. The correct answer should state that the warm front advances warm air, has a gently sloping boundary, and moves in an eastward or northeastward direction.