The idea that different pairs of genes are passed to offspring independently is Mendel's principle of:
Inheritance
Linkage
Segregation
Independent assortment
As discussed in the Law of Independent Assortment -
Independent assortment states that the genes that determine a trait assort independently of the genes for other traits. As a result, new combinations of genes, present in neither parent, are possible and therefore, different pairs of genes are passed to offspring independently. Hence, the correct option is (d).