He was in trouble because he did not . . . . . . . the instructions.

The correct answer is A. follow.

To follow means to do what someone tells you to do. In this case, the person was in trouble because he did not do what the instructions told him to do.

The other options are incorrect because they do not mean to do what someone tells you to do.

• Observe means to watch something carefully.
• Confirm means to check that something is true.
• Chase means to run after someone or something.