No issue with any of these names being allowed as alternative answers, but The Plough (a.k.a. the Big Dipper or Charles's Wain) is not a constellation; it's more correctly referred to as an asterism. Wikipedia defines an asterism as "a popular known pattern or group of stars that are recognised in the night sky ... very similar to a constellation, but they differ mostly in that a constellation is an officially recognized area of the sky, while asterisms do not have officially determined boundaries."
In modern times, this particular asterism tends to be known as The Plough in the British Isles and the Big Dipper in North America; anyone who answered "Charles's Wain" would be having a laugh, or (to put it less politely) just showing off.
The name "Charles's Wain" goes back at least to the 16th century. Brewer's Dictionary of Phrase and Fable gives it as "an old popular name for the seven bright stars of the Great Bear", and says that "it almost certainly derives from Charlemagne's wain." (A 'wain' is a cart or wagon.) Richard Hinkley Allen, writing in 1963 (and reproduced by Bill Thayer on the University of Chicago's website) quotes an entry from the New English Dictionary, according to which the name arises from the legendary association between the Holy Roman Emperor Charlemagne and King Arthur of Britain. This appears to arise from the confusion of Arcturus with Arthur. Arcturus is the brightest star in the constellation Boötes (the ploughman or herdsman); its name actually means 'guardian of the bear'.
Arcturus is the fourth brightest star in the night sky, as seen from Earth, and the brightest in the northern celestial hemisphere. Boötes is to the left of The Plough (the 'handle' of the Plough points towards it).
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