Final answer:
Chromosome rearrangements, particularly deletions, are the class of chromosome mutations that lead to a decrease in genetic material within a cell. Aneuploidy can result in either an increase or decrease of chromosomes but is not a deletion, while polyploidy increases genetic material.
Step-by-step explanation:
The class of chromosome mutations that result in a decrease in the amount of genetic material in a cell is termed chromosomal rearrangement. Specifically, a deletion, which is one type of chromosomal rearrangement, occurs when a segment of DNA is lost from a chromosome. This can lead to the loss of genetic information as genetic material is physically removed from the chromosome, often resulting in numerous genes missing from the chromosome. Contrary to deletions, aneuploidy can either increase or decrease the amount of genetic material depending on whether there is a loss or gain of chromosomes. However, the action of aneuploidy alone does not necessarily decrease genetic material. Polyploidy, on the other hand, results from the duplication of entire sets of chromosomes and would lead to an increase rather than a decrease in genetic material. Therefore, the correct answer is chromosome rearrangement, which includes deletions that can lead to a reduction in genetic material within a cell.