Final answer:
The receptor cells active in dim light are the rods, which are sensitive to low levels of light and are crucial for seeing shades of gray, whereas cones are responsible for color vision in brighter light.
Step-by-step explanation:
The sensory receptor cells that are active in dim light are the rods. When comparing rods and cones, which are the two major types of photoreceptors located in the retina of the human eye, rods are highly sensitive to light and are responsible for vision in low-light conditions. They do not detect colors but are responsible for perceiving different shades of gray. Rods contain the pigment rhodopsin, which is very sensitive to light, enabling us to see in dimmer environments. On the contrary, the cones are less sensitive and are responsible for color vision and function well in bright light conditions. So, the correct answer to the question is option (b) Rods.
Furthermore, cones are concentrated in the fovea, the area of the retina responsible for high acuity vision, and operate best in daylight or well-lit conditions. In contrast, rods are distributed throughout the retina and contribute to peripheral vision and motion detection. Given the differences between rods and cones, statement (a), asserting that rods detect color while cones detect only shades of gray, is false. Also, statement (c) is indeed true; the iris is the part of the eye that adjusts the light level allowed into the eye.