About Lighting and reflection of the trailer: which statement is WRONG?

The red TRIANGULAR reflectors belong at the REAR; at the front are white (non-triangular) retroreflectors.
White retroreflectors belong at the FRONT; at the rear are red triangular reflectors.
✓ A rear fog light is optional on a trailer.
This trailer itself must always have a rear fog light.

Correct answer: A rear fog light is optional on a trailer.

Explanation

Red rear fog light is in the trailer's minimum equipment requirements; it is always required, not optional.

The red TRIANGULAR reflectors belong at the REAR; at the front are white (non-triangular) retroreflectors.
White retroreflectors belong at the FRONT; at the rear are red triangular reflectors. Red rear, white front.
This trailer itself must always have a rear fog light. The only exception is if the TOWING vehicle itself has no fog light. The law does contain a date nuance for older heavy trailers, but the exam follows the book: treat the rear fog light as always mandatory.