Which of the following statements is true for scientific theories and laws?

Essentially laws and theories are the same thing. In modern day science we don't generate "laws" per se, we call highly tested and sound hypotheses "theories" rather than laws. As such, both things can (and should) be questioned if there is repeatable evidence to support a contradiction. This is what keeps science moving forward.
For example we have Newton's Laws of motion which were shown to be only approximations by the theory of relativity. These were long held as true Laws of Nature but are actually incorrect.

The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.