Some grammar books teach this:
But note:
There are three chairs and a table in the room.
There’s a table and three chairs in the room.
The general rule is that the verb form matches the item(s) that it is adjacent to*:
https://www.bbc.co.uk/worldservice/learningen...
*This is a simple explanation designed for learners, especially younger learners - a "general rule" that does not apply in all situations.
Informal English means relaxed, everyday English you use in the street, with your friends or family. In informal English, we often use the short form "there's" with a plural, so we would say "There's two rabbits!" or "There's a cat and two rabbits!". But the long form "There is a cat and two rabbits!" doesn't sound as good - it sounds like bad English.
Formal English means careful English that you use in important situations - e.g. an academic essay or an official report. In formal English, we don't use the short form "there's", so we would rewrite the sentence, e.g. "There are two rabbits and a cat".
So an exam in the UK would not have "There is a cat and two rabbits" as the correct answer.
I agree that this exam question is badly written if the "correct" answer is "there is a cat and two rabbits".