What supports the top of the walls and prevents the weight of the roof from pushing the walls apart?

The top plate is the component that supports the top of the walls and plays a crucial role in maintaining the structural integrity of the building. It runs along the top of the wall studs and serves as a connection point for the roof framing. The top plate helps to distribute the load from the roof structure, preventing the weight from causing the walls to spread apart.

In contrast, ceiling joists primarily support the ceiling and transfer loads to the walls but do not provide lateral support to keep the walls from pushing outwards. The ridge board is situated at the peak of the roof and serves as a horizontal support for the rafters, but again, it does not offer the same lateral support that a top plate provides. Rafters help support the roof and transfer loads down to the walls, but they do not prevent the walls from moving apart; that function is specifically served by the top plate.

Thus, the top plate is the correct answer as it fulfills the role of stabilizing the wall structure against the loads imposed by the roof system.