Which of the following is a common cause of battery drain in vehicles?

A faulty alternator is a common cause of battery drain in vehicles because the alternator is responsible for charging the battery while the engine is running. If the alternator is not functioning properly, it may not adequately replenish the battery's charge after it has been used to start the engine or to power electrical components. As a result, the battery can become depleted over time, leading to situations where the vehicle might fail to start or exhibit electrical issues.

Understanding the other options provides clarity on why they are less relevant to the issue of battery drain. A faulty starter motor typically affects the initial starting process of the vehicle rather than draining the battery over time once the engine is running. Worn brake pads relate to the braking system and do not impact battery performance. An empty fuel tank primarily affects engine operation but has no direct correlation with battery charge levels. Therefore, the alternator is a critical component when considering the overall health of a vehicle's electrical system and its ability to maintain battery charge.