What did Fred do while driving on the highway?

The choice indicating that Fred "kept on" while driving on the highway suggests that he maintained a steady pace or continued moving forward without interruption or reduction in speed. This implies a sense of consistency in his driving behavior, which is often expected or necessary on highways where constant movement is typically required for safety and traffic flow.

Choosing this option reflects an understanding of highway driving dynamics, as drivers usually aim to keep up with the flow of traffic. In the context of the other options, they may imply specific actions that could either interrupt the flow of driving, such as stopping or changing lanes frequently, or imply a reduction in speed, which is not consistent with the notion of maintaining smooth progress on a highway. Thus, "kept on" correctly captures the essence of driving behavior that prioritizes consistent movement on such roadways.