Final answer:
Pluto may be mistaken for an asteroid or comet due to its small size, and it's located in a celestial neighborhood with similar objects. 'By mistake' is a synonym for 'accidentally'. Observations of Charon from Earth are difficult as it is close to Pluto and has a similar orbit.
Step-by-step explanation:
The reasons people may think Pluto is an asteroid or comet are varied, but option (d) 'it is so small' is a likely reason because Pluto's size is not comparable to that of other planets; it's significantly smaller and shares more characteristics with smaller celestial bodies. Another possible contributing factor is that, like asteroids and comets, Pluto is found in a region of the solar system known as the Kuiper Belt, where many other small, icy objects reside.
Some alternative words to 'accidentally' include 'by mistake' (option b), as it implies something was done unintentionally, which is a synonym of 'accidentally'.
Concerning Charon, Pluto's largest moon, it's difficult to distinguish it as a separate body when observing from Earth due to its proximity to Pluto and because it shares a similar orbit, making the two appear very close together.