Which term best describes the type of reaction that happens when a single element reacts with a molecule or compound producing a new single element and a new molecule or compound? a) an isolation reaction b) a single combustion reaction c) a single replacement reaction d) a double replacement reaction

The answer to this question would be: C) a single replacement reaction

In single replacement reaction the molecule made in just one with an element. In this case, the compound is just swapping its element with a more reactive one, results in a more stable compound.
Contrast to the single replacement reaction, double replacement reaction or metathesis is the reaction which results in two new compounds. The cation from the molecule reaction is switching with the cation from other molecules. 
In contrast to the double replacement reaction, 

The correct answer is "atomic weight".