Change to passive voice: He was driving a car when an accident occurred.
AA car was being driven by him when an accident occurred.
BA car had been driven by him when an accident occurred.
CA car was driven by him when an accident occurred.
DA car is being driven by him when an accident occurred.
Correct answer
A. A car was being driven by him when an accident occurred.
Explanation
The past continuous passive uses was/were being followed by the past participle.