Mendel Correctly Surmised That the 3 Dominant:1 Recessive Phenotypic Ratio

Mendel correctly surmised that the 3 dominant:1 recessive phenotypic ratio seen in the F₂ generation pea plants was due to

A) a tendency to have more fertilization events yielding homozygous dominant than homozygous recessive.
B) homozygosity.
C) genotypic selection.
D) random combination of sperm and egg with respect to the allele carried.
E) certain phenotypes having a selective advantage.