What type of cells contains nuclei with one set of chromosomes?

Haploid cells contain a single set of chromosomes, which means they have one copy of each chromosome instead of the paired sets found in diploid cells. This characteristic is evident in gametes, such as sperm and egg cells in animals, where the reduction in chromosome number is crucial for sexual reproduction. When two haploid gametes fuse during fertilization, they form a diploid zygote with two sets of chromosomes, one from each parent. The simplicity of having just one set of chromosomes allows for the regulation of gene expression that is essential for the development of sexually reproducing organisms.

The other types of cells mentioned differ in their chromosome number: diploid cells have two sets (one from each parent), polyploid cells have more than two sets, and euploid refers generally to cells with a complete set or multiples of the standard number of chromosomes, which can include both diploid and polyploid cells. Therefore, haploid specifically designates cells characterized by having one complete set of chromosomes, making it the correct choice.