What are the variables in her hypothesis

A hypothesis states a presumed relationship between two variables in a way that can be tested with empirical data. It may take the form of a cause-effect statement, or an "if x,...then y" statement. The cause is called the independent variable; and the effect is called the dependent variable.

b. the carbon cycle ensures living things have a source of hydrogen

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.