The next step Fiona should take in using the scientific method to answer the question "Do oxygen levels in the air affect rates of photosynthesis?" is:
Form a hypothesis.
Forming a hypothesis is an essential step in the scientific method. A hypothesis is a testable and falsifiable statement that predicts the relationship between variables in an experiment. In this case, Fiona should develop a hypothesis that suggests whether or not oxygen levels in the air have an impact on the rates of photosynthesis.
For example, Fiona's hypothesis could be:
"Hypothesis: If the oxygen levels in the air increase, then the rate of photosynthesis in the plants will also increase, as oxygen is one of the key byproducts of photosynthesis."
After forming her hypothesis, Fiona can proceed to design a procedure for her experiment, where she will set up controlled conditions to test the validity of her hypothesis. The scientific method involves a systematic approach, and each step builds upon the previous one, leading to the collection and analysis of data to draw conclusions about the question being investigated.