When plants are designed to contain more sets of chromosomes than normal, which of the following have humans taken advantage of?
a. hybridization
b. polyploidy
c. inbreeding
d. selective breeding

D.
Selective breeding

In selective breeding, the desired genes from one organism are combined with genes of another organism, resulting in a new combination of genes. 
The selective breeding is quite voluntary and is not necessarily natural or congenital. It is the act of how people or individuals can choose traits in the gene pool of their choice  to produce their desired or goal organism in the process. This trait is influenced in the host of the specific sperm and egg cell which makes up the chromosomes.