Final answer:
High-density lipoproteins (HDL) are known as "good cholesterol" because they transport cholesterol from the tissues back to the liver, helping to prevent atherosclerosis and therefore are good for the heart.
Step-by-step explanation:
High-density lipoproteins (HDL) are beneficial for heart health; therefore, the correct answer to the question is C. are good for the heart. HDL is known to scavenge cholesterol from the body tissues and transport it back to the liver, where it can be processed and removed from the body. This role in cholesterol transport helps to prevent the buildup of plaque in the arteries, a condition known as atherosclerosis, which is a risk factor for heart disease. HDL is made up of a significant proportion of protein, around 33%, and the rest is lipids, mostly phospholipids and cholesterol, but not solely triglycerides as suggested in one of the wrong options.