You obtain an spo2 reading of 100% on a patient receiving oxygen via a nonrebreathing

SpO2 measures blood oxygen saturation levels. Normal is between 95-99%, and the only way to achieve 100% is with supplemental oxygen. Once you know the SpO2 value, you can estimate the arterial PO2 value using the 4-5-6-7-8-9 rule. For example:
  SpO2 70% = PO2 of 40
 SpO2 80% = PO2 of 50
  SpO2 90% = PO2 of 60
 SpO2 100% (our patient) = PO2 of 70.
  If the SpO2 were to drop a bit from 100%, the PO2 would also drop a bit. The range of possible PO2 values in our patient is: 60-70.

The second option is the best answer.

Proteins are complex polymers of amino acids. They are folded into 3 dimensional structures . This folding defines not only the size and shape of the protein, but also its function.

The folding of a protein allows for interactions between amino acids that may be distant from each other in the primary  sequence of the protein. In enzymes, some of these amino acids form a site in the structure that catalyzes the enzymatic reaction. This site is called the active site.