Human heart cells require a constant release of energy to function continuously. which structure would be a numerous in such cells to meet this demand?

The Mitochondria.

Muscle cells have numerous mitochondria, more than other types of cells, so that they can quickly and forcefully respond to the body's constant need for various types of movement.

Mitochondria

Explanation:

The heart muscles require energy to work. The energy that is required by the body to perform various functions is fulfilled by adenosine triphosphate.

The production of adenosine triphosphate takes place in the mitochondria. It is the site where the conversion of glucose into high energy molecules takes place.

The region of the body that requires more energy has more mitochondria.So, if the heart needs energy it will require more mitochondria.

the answer would be d