When Ruby practice piano she gets a sticker, and when she does a chore she gets 2 stickers. When she has 20 stickers her parents will buy her a
pizza as reward. This system is called?

A reward system.

Explanation:

My parents use this system all the time and they say that it is called a reward system.

explanation:

with the profits from the european sales, merchants purchased europe's manufactured  goods, including tools and weapons. then the merchants shipped those manufactured  goods, along with the  american  sugar and rum, to west africa where they were bartered for  slaves.