Which example correctly describes the relationship of form and function at the chemical level?

the binding of chemical messengers to receptors often results in a shape change, which affects receptor function.
receptors on plasma membranes maintain their shape whether they are bound or unbound to chemical messengers.
the chemical messengers act only on the cells that release them.
cellular receptors exist only on the outer surface of the plasma membrane.

The binding of chemical messengers to receptors often results in a shape change, which affects receptor function.

Explanation:

Binding of specific chemical messengers to the corresponding receptors of target cells brings about change in the shape of the receptors. This structural change in the receptor results in the generation of the desired response. It reflects how the form and specific configuration of substances or biomolecules affect their functions.

For example, binding of neurotransmitters to the receptors present on neurons opens the associated ligand-gated ion channels to allow the movement of specific ions through it. In the absence of bound ligand, these channels are present in a closed conformation.  

when a reversible reaction is occurring in both directions at the same rate; the products and reactants do not change in concentration. a chemical reaction where the compound is broken into its constituent parts.

your body is trying to warm itself up. or more precisely, sensors in your skin send messages to your brain to generate heat to your body.