What is an open line of sight?

An open space in front of the driver

A line of objects which can be seen by the driver

This is the ability to see the center of the driver's path of travel

Correct answer: "This is the ability to see the centre of the driver’s path of travel"

Considering the driving context, an open line of sight is what you can see of your path of travel from your vehicle to where you're heading. The line of sight can be blocked by how the traject is, for example, the presence of a hill's crest, a curve, a large vehicle in ahead, and others.