Me . i really don't know the answer to this.

Your dependent variable "depends" on the independent variable. In this question your dependent variable is going to be plant growth. Your plant growth "depends" on the amount of sugar in the water. 

Therefore, your independent variable is the amount of sugar in the water. 

I hope I was able to help. Best of luck.

The amount of water if you write a sentence or it gives you the option that should be the amount of water