Step 1: Understand the term "selectivity."
Selectivity is a measure of how effectively a reactant is converted into the desired product, as opposed to an undesired product. It is the ratio of the moles of the reactant converted to the desired product to the moles of the reactant converted to undesired products.
Step 2: Conclusion.
From the given options, the correct term that describes this ratio is selectivity.
Final Answer: The ratio is called selectivity.