Answer:
Just after waking up in the morning
Step-by-step explanation:
A good time to check it is in the morning after since before that you were sleeping. But when you check it you should make sure to check it BEFORE you get out of bed.
So basicially, the best time to check your resting heart rate as you stated is when you wake up in the morning before you get out of bed. Which means that the answer is D.
I know I wrote a lot sorry, i just wanted to give a good explanation
Hope this helped a little tho.