Which of the following is a common symptom of a failing battery?

A common symptom of a failing battery is the slow cranking of the engine. When a battery begins to fail, it may not hold a sufficient charge to provide the electrical power needed to start the engine efficiently. As a result, the starter motor struggles to turn the engine over, leading to a slower cranking speed. This can be particularly noticeable in colder weather when batteries are less effective, but it can happen at any time if the battery is worn out or defective.

The other options, such as increased fuel efficiency, higher engine RPM, and improved ignition timing, are not symptoms of a failing battery. In fact, a failing battery can lead to issues with engine performance, but they would not manifest as improvements in fuel efficiency or engine speed. Understanding how a battery impacts the starter motor and overall engine operation is crucial for diagnosing starting issues in a vehicle.