Final answer:
Planetary systems (option c) haven't been directly observed around stars other than the sun primarily because planets are small and emit light only by reflection, making them hard to distinguish from their much brighter parent stars, rather than due to their rarity or the distance of other stars.
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 are hard to detect as they are small and shine by reflected light, which is a tiny fraction of the light emitted by their parent star. Furthermore, the reflected light from the planets is usually swamped by the brilliant glare of their much brighter parent stars, making them difficult to observe directly from our distant vantage point.
However, technological advancements have enabled astronomers to detect the indirect effects of these exoplanets on their stars, despite the challenges posed by the star's brilliance and the planets' relatively small size. We have been able to measure changes in a star's motion caused by a planet's gravity, or the decrease in starlight when a planet transits in front of the star.