Which of the following is a common supplementary light source used by photographers?

A separate flash unit is indeed a common supplementary light source used by photographers. This equipment provides additional illumination, helping to brighten scenes and reduce shadows in various lighting conditions. Flash units can create a more balanced exposure, particularly in circumstances where natural light is insufficient or when a photographer wants to highlight specific subjects or elements within a composition.

Using a separate flash unit allows for control over the intensity and direction of the light, making it versatile for different scenarios such as portrait photography, event coverage, or product shoots. This additional flash can help replicate the effect of natural lighting or create dramatic effects by manipulating shadows.

Reflectors and softboxes also serve as light sources; however, they modify existing light rather than provide independent illumination. Reflectors bounce available light to enhance brightness, while softboxes diffuse artificial light sources for a softer, more flattering effect. Meanwhile, a light meter is a tool for measuring light intensity, but it does not emit light. Each of these tools plays a role in lighting, but the standalone function of a separate flash unit as a supplementary light source distinguishes it as the correct answer.