Which of the following is a requirement for amber reflectors placed on the sides of certain large vehicles?

Amber reflectors on the sides of certain large vehicles serve the critical purpose of enhancing visibility and safety. The requirement for these reflectors to be approximately one-third the length of the side of the vehicle ensures they are adequate in size to attract attention without being obscured or indistinguishable among the vehicle's design. This specification assists in ensuring that other drivers can easily recognize the dimensions and presence of the large vehicle, particularly in low-light conditions or during adverse weather.

The other options do not meet the specific requirement for side-mounted amber reflectors. For instance, the positioning of reflectors at the front or rear does not pertain specifically to the requirement for side reflectors, which are meant primarily for lateral visibility. While reflectors should be visible during daylight, this has more to do with placement and brightness rather than the defined size regulation. Therefore, the correct choice emphasizes the proportional aspect of the reflector in relation to the vehicle’s side, which is crucial for effective visibility and safety.