What is the most common cause of collisions?

Answer: B — Driver error

Collisions can occur due to various factors, but the most common cause is driver error. This is because a large majority of collisions are a result of drivers not following traffic laws or making careless mistakes, such as distracted driving or speeding. Defective vehicles may also contribute to collisions, but they are not as common as driver error. Bad roadways and poor weather conditions can also play a role in collisions, but they are ultimately less common and can often be avoided by following safe driving practices. Therefore, the correct answer is driver error.