The length visible to driver at any instance of time is called ___________a.Sight distanceb.Visibility limitc.Head light distanced.Stopping sight distance

The correct answer to the question is a. Sight distance.

Explanation:

Sight distance refers to the length of the road visible to a driver at any given moment, which is crucial for safe driving. It encompasses the information a driver can obtain about the road ahead, including obstacles, vehicles, and changes in the roadway. Sight distance is influenced by multiple factors, such as the height of the driver's eye, the condition of the road, and any obstructions like trees or buildings. Proper sight distance allows drivers to react appropriately to changes in traffic conditions, thus enhancing road safety.

Understanding sight distance is vital when designing roadways, as inadequate sight distance can lead to accidents. It's also important in assessing visibility during nighttime driving, where headlight capabilities play a significant role in determining a driver's sight distance.

In contrast, the other options either do not specifically define the driver’s visible length of road or refer to different concepts related to driving and visibility. Hence, it confirms that the best fit for the blank is "sight distance."