B1
If an action or decision is up to someone, they are responsible for doing or making it.
Dictionary examples:
It's up to you to choose: broccoli ice cream or pickle-flavored cookies? (0.0)
It's up to you to figure out why your socks keep disappearing in the wash. (0.0)
It's up to Bob to explain why he's wearing a banana costume. (0.0)
I can't decide for you, Jack, it's up to you. (26.1)
It's up to Sophie to decide whether she wants to enter the competition. (34.2)