Final answer:
The correct statement that indicates an understanding of the teaching is option C: "I will contact my doctor if the medication gives me a headache."
Step-by-step explanation:
In this scenario, the correct statement that indicates an understanding of the teaching is option C: "I will contact my doctor if the medication gives me a headache."
This statement shows that the client understands the potential side effects of taking nitroglycerin, such as headaches. It is important to notify the doctor if any adverse reactions occur.