Levi decides to examine the effect of fertilizer on the growth of tomato plants. He chooses four plants for his experiment and applies varying amounts of fertilizer to three of them. He does not apply fertilizer to one plant. Over a 15-day period, the plants receive fertilizer on Days 1, 4, 7, 10, and 13. Levi measures the height of all of his plants with a meterstick on Days 3, 6, 9, 12, and 15. He also makes sure to hold all experimental factors constant except for the fertilizer.

What is the independent variable in Levi's experiment on tomato plants?

the days plant height was measured

the days fertilizer was applied

the amount of fertilizer given to the plants

the measurements of plant height