Final answer:
C) Planetary systems are not observed directly around other stars mainly because the light planets reflect is too faint compared to their stars' brightness, and not because planetary systems are rare.
Step-by-step explanation:
The least likely reason why planetary systems have not been directly observed around stars other than the sun is that planetary systems are rare.
This is because planets themselves are difficult to detect as they are illuminated only by reflected light from their parent stars, and this light is minuscule compared to the brightness of the stars themselves.
Planets are also relatively small and they get lost in the brilliant glare of the stars they orbit. We have been able to discover exoplanets primarily through indirect methods.
Such as observing the gravitational effect they have on their star or the dimming that occurs when they transit in front of their star. The use of microlensing has also been explored as a technique to detect planets by utilizing the bending of light due to gravity.