What would cause the percent yield of a reaction to be less than 100%?

The product participates in a side reaction.

A balanced chemical equation is used.

All of the limiting reagent gets used up.

Pure reactants are used.

first one is correct

All of the limiting reagent gets used up causes the percent yield of a reaction to be less than 100%

Explanation:

It is rare for a chemical reaction to occur with the right proportion of reactants that reacts together to form resultant products. During this process, one of the reactants gets used up faster resulting in the lower percent yield. Such product that gets used up faster is called limiting reagent. This limiting reagent is responsible for lowering the percentage yield in the chemical reaction.

The limiting reactant can be found easily by calculating the yield of each reactant assuming they are consumed completely. The reactant which has least yield is the limiting reactant of the reaction.