Final answer:
In the dark, Marcel's rods will be the most active photoreceptors, as they are responsible for low-light vision and can detect shades of grey.
Step-by-step explanation:
When Marcel is trying to find his keys in the dark, the type of photoreceptor that will be most active in this situation is a) Rods. Rods are significantly more sensitive to light than cones and are crucial for vision in low-light conditions. Unlike cones which are responsive in bright light and essential for color vision, rods are responsible for vision at low light levels and can only detect shades of grey. Since Marcel is in the dark, his rods are the ones that will be enabling him to see.