Which molecule in this figure portraying water-soluble hormone action is the receptor protein? 
Definitions:
National Defense
Governmental and military actions and policies aimed at protecting a country from external aggression, threats, and maintaining national security.
Patented Invention
A unique product or process that has been legally protected by a patent to give its inventor exclusive rights to use, make, or sell it for a certain period of time.
Tornado Siren
A warning system used in many communities to alert citizens about the imminent danger of a tornado.
Excludable
A characteristic of a good or service that allows the owner to prevent others from using it without permission.
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