Question

hormones binding to receptors
target cells make more receptors for a particular hormone when the level of that hormone declines in the bloodstream. this process is known as
synergism
the second messenger system
down - regulation
up - regulation

Explanation:

Synergism refers to hormones working together. The second - messenger system is about intracellular signaling. Down - regulation is when receptor numbers decrease. Up - regulation is when target cells increase receptor production in response to low hormone levels.

Answer:

up - regulation