What must you do if an officer instructs you to take an alcohol test while driving?

Answer: C — Take the test as it is required by law

If an officer instructs you to take an alcohol test while driving, it is important to comply and take the test as required by law (Choice C). Refusing to take the test can lead to serious consequences, such as license suspension or revocation. It is necessary to follow the officer's instructions in this situation to avoid legal repercussions. Therefore, taking the test as required by law is the correct course of action.