In which scenario is it necessary to signal a passing intention with headlights?

Signaling a passing intention with headlights is particularly important when passing another vehicle, primarily for safety and communication purposes. Using headlights to signal communicates to other drivers your intentions, helping to reduce the likelihood of misunderstandings or accidents when attempting to overtake. In situations where visibility may be compromised or when the driver being overtaken may not be aware of your presence, signaling ensures that everyone is informed of your actions on the road.

In scenarios such as heavy traffic, while signaling may be relevant, it is not specifically required to indicate a passing intention. Likewise, signaling only when another driver is visible does not encompass all scenarios in which passing occurs. Adverse weather conditions may necessitate increased caution on the road but do not specifically call for signaling a passing intention; therefore, these instances are not as directly related to the necessity of signaling when passing another vehicle as the correct answer is.