C. A is incorrect because 'grow better' is very broad, doesn't really tell us what you're assessing - leaf size, plant height etc, and also doesn't test the effect of different lights because it is only testing red. B doesn't say anything about different coloured lights. D isn't testing different coloured lights, it's testing the effect of one light on two plants.
C, however, tells you what it is testing (height), and that there are two types of light, and thus is testing the effect of light on plant growth. does that make sense?