Sentences a, b and c mean literally what they say. They are not idioms or idiomatic expressions.
As for, “Let’s play it by ear”, it means “Let’s play this music without reading the musical notation.” (assuming, of course that the speaker is playing a musical instrument and listening to some kind of music which he/she wants to play).
I'm thinking that B is the closest because it is saying that there is no need to decide right now. We can see how things develop and then decide, improvise as needed.