What are the non-substrate molecules binding to the allosteric sites called?

The correct option is allosteric modulators.

Let's break down the explanation step-by-step to understand why this is the correct answer:
An enzyme typically has an active site where the substrate binds to undergo a chemical reaction. However, many enzymes also possess alternative binding sites located away from the active site, known as allosteric sites.

Molecules that bind to these allosteric sites are not the primary substrates of the chemical reaction. Instead, their binding induces a conformational change (a change in the shape) of the enzyme. This structural shift alters the active site's affinity for its substrate, thereby regulating (either increasing or decreasing) the enzyme's catalytic activity.

Because these non-substrate molecules modulate or regulate the enzyme's function by binding to the allosteric site, they are collectively referred to as allosteric modulators (or allosteric effectors). They can act as allosteric activators (which enhance activity) or allosteric inhibitors (which decrease activity).