Which of the following represents a secondary reinforcer?

A secondary reinforcer is a stimulus that has acquired its reinforcing properties through association with a primary reinforcer. In this case, toys represent a secondary reinforcer because they are not inherently rewarding but often become desirable through their association with satisfying needs or desires, such as play and enjoyment.

In contrast, a loud siren, food, and shock do not reflect this distinction in the same way. A loud siren could be considered an aversive stimulus rather than a rewarding one. Food, conversely, is a primary reinforcer as it fulfills biological needs. Shock is typically viewed as an aversive stimulus designed to decrease undesired behaviors and does not serve as a reinforcer in the positive sense. Therefore, toys, as secondary reinforcers, underscore the idea that their value comes from learned experiences and associations rather than intrinsic properties.