According to regulations, which condition prohibits a pilot from flying?

A pilot is prohibited from flying when taking medications that affect their faculties because these substances can impair cognitive and motor skills critical for safe flight operations. The regulations emphasize the importance of a pilot's ability to perform tasks and make sound judgments, which can be compromised by certain medications. Regulators require pilots to ensure their medical fitness for flying and specifically list certain types of medications that may lead to disqualification.

In contrast, while a light hangover, moderate fatigue, and low blood sugar can certainly impact a pilot's performance, the regulations more explicitly address the use of medications as a direct prohibition due to the known impairments they cause. Therefore, a pilot can be deemed unfit to fly if they are under the influence of medication that negatively impacts their abilities, making this the basis for the correct answer.