Which one of the following options is an example of a systemic fluoride application?

Systemic fluoride application involves the ingestion of fluoride, allowing it to become incorporated into developing teeth and affecting overall dental health. Fluoride drops are a systemic form because they are taken internally, typically by children, to provide fluoride to the body, which is then distributed throughout the bloodstream and incorporated into the mineral structure of the teeth.

In contrast, the other options, such as fluoride mouthwash, toothpaste, and varnish, are primarily topical applications. They work by directly applying fluoride to the surfaces of the teeth but do not involve ingestion or systemic absorption. These topical methods help in remineralizing the teeth and inhibiting decay but do not provide the systemic benefits that fluoride drops do, such as strengthening the teeth during their development. Thus, fluoride drops represent the correct choice as an example of a systemic fluoride application.