When you press on the gums, release the pressure, and then measure the time it takes for the gums to return to the starting color, you're checking

Capillary refill time

Explanation:

Capillary refill time refers to the time that it takes for color to return to an external capillary bed after pressure has been applied (which causes blanching). Usually, on healthy individuals, capillary refill time takes less than 2 seconds. If the time is much longer, this could indicate problems such as shock, dehydration or peripheral artery disease.

i believe it would be the lungs.

c is the correct answer hope this !