What is 'clutch riding' (riding the clutch) in manual vehicle operation, and why should it be avoided?

1. Keeping the foot resting lightly on the clutch pedal while driving, which causes premature wear on the release bearing and friction plate
2. Using the clutch pedal to accelerate without pressing the gas pedal
3. Pressing the clutch pedal fully down when stopping at a red light
4. Shifting gears without using the clutch pedal at all