Which of these is most like a closed system that does not allow transfers in or out of the system?
a planet earth
b a coral reef
c a school
d the human heart

Planet Earth  is most like a closed system that does not allow transfers in or out of the system

A. Planet Earth

Explanation:

In a closed system, there is no exchange of mass. However, the energy might get exchanged. The exchange of mass and energy depends upon the property of the membrane or the boundary of the substance or an object.

The Earth is basically a closed framework; it acquires huge amount of energy from the Sun yet the exchange of matter with the outside is just about zero. Material streams between stores or sinks along pathways, or motions.

so is it d c or e