If the sentence "The wheels on every car were also inspected before each journey by using a hammer to tap each wheel to listen for irregularities" were written in active voice, it would likely specify who is performing the action of inspecting the wheels. Thus, the correct answer is:
D. It would specify who inspected the wheels.
In the passive voice, the subject performing the action (the person or entity inspecting the wheels) is not mentioned, whereas the active voice would clarify this information.