The correct answer is - A- Amount of nitrogen in fertilizer.
The independent variable, in mathematical terms, is the variable whose variation is not dependent on the variations of the another variables.
In this case, the independent variable will be the amount of nitrogen in the fertilizer, as it is not dependent on any other variable. Also, apart from not being dependent on other variables, it is a variable on which the other variables are dependent, which in this case will be the amount of leaves and the amount of blossoms on the tomato plant.