It didn't ask for 'this bus', just 'the bus'...
Because it is not talking about buses in general, その is here used to make this definite, specific bus. I disagree with the usage though unless the bus is near enough the listener and the speaker to be labeled as "the bus"
But Duolingo has (so far) been happy to translate as "the X" something that is not marked with a demonstrative. "The cat is....". Surely 猫は小さいです can be translated as "the cat is small"?
If it's a particular bus, shouldn't it be が? Or do I have them mixed up?
Could it not be ano basu... instead of sono ?
do the japanese have the same sentence structure as yoda? "This bus, small is"