It's a piece of cake is all about metaphors and similes.
Your own first language probably has many colourful expressions for everyday, and less usual, situations. Can you identify which ones we use in such circumstances, in English? 'It's a piece of cake' ~ i.e. easy, pleasant, even energy- and confidence-boosting!
'A bull in a china shop' conjures up the image of 'an accident waiting to happen' (as we also say; think about that!).
Answers 3 and 4 also suggest unsuitable and/or futile things, but each of these is more usually used to describe things that drop or sink, including metaphorical uses such as when a supposedly persuasive performance fails to convince anyone (e.g. a speech or drama segment 'goes down badly'), or where a number of singers without instrumental accompaniment begin to slip off-key and 'lose pitch' (usually, downwards).