The answer is C. Cluster.
A collection is a group of things that are gathered together. A compilation is a collection of things that have been put together in a particular order. An assemblage is a group of things that are put together in a particular way. An assortment is a group of things that are different from each other. A cluster is a group of things that are close together.
In this case, the word “cluster” is different from the rest because it is the only word that refers to a group of things that are close together. The other words all refer to groups of things that are gathered together or put together in a particular way.