Kelp plant have been known to grow up to a. 1 foot per week b. 2 inches per day c. 10 inches per day d. 1 inch per hour

Kelp plants have been known to grow up to C. 10 inches per day. 
It really depends of how much sunlight they get, because these rays help kelp plants produce food and the necessary nutrients. With the help of sunlight, these plants can grow faster.