Most steering wheel hubs have​ _______ splines.

The correct option is that most steering wheel hubs have internal splines. Internal splines are designed to fit onto the steering shaft, allowing for a secure connection that helps transmit the driver's steering input to the steering mechanism effectively. This design enhances the reliability of the vehicle's steering system by ensuring that the steering wheel can turn smoothly without slippage.

On the contrary, external splines would not fit onto the steering shaft in the same way, as they are designed to slide over a shaft rather than being fitted inside it. Hypoid refers to a type of gear, which is irrelevant in the context of steering systems. Diagonal does not pertain to the spline configuration generally found in steering wheel hubs. Therefore, internal splines are the standard configuration used to ensure effective steering performance in vehicles.