Answer:
Science Question: When the plant is exposed to light for longer periods of time, it's height increases more than those plants that weren't exposed to light for as long.
Control: I think the control would be a plant that was exposed to light for 24 hours (all day). Not completely sure, but that's what i would say. (If you aren't sure what a control is, it's just a sample that you don't change at all. It's used to compare other samples' results.)