Regular method or order; formal arrangement; plan; as, to have a system in one's business.
2. an assemblage of objects arranged in regular subordination, or after some distinct method, usually logical or scientific; a complete whole of objects related by some common law, principle, or end; a complete exhibition of essential principles or facts, arranged in a rational dependence or connection; a regular union of principles or parts forming one entire thing
3. hence, the whole scheme of created things regarded as forming one complete plan of whole; the universe.
4.the collection of staves which form a full score.
which is the best meaning for system as it is used in this sentence?

heatstroke occurs when the body’s internal temperature becomes so elevated that its cooling system begins shutting down.
a) definition 1
b) definition 2
c) definition 3
d) definition 4