Explanation:
A dihybrid cross involves the study of inheritance of two different genes, each located on a different pair of homologous chromosomes. The F1 generation resulting from the cross between two pure-breeding parents (homozygous for two different traits) will be heterozygous for both traits. When these F1 individuals are allowed to self-fertilize or cross-fertilize with each other, the resulting F2 generation will exhibit different phenotypic ratios depending on the mode of inheritance of the two traits.

The phenotypic ratio of F2 generation in a dihybrid cross can be determined by using the Punnett square method or by applying the principles of probability.

In the Punnett square method, the alleles of one gene are written on the top of the square, and the alleles of the other gene are written on the left side of the square. The possible gametes produced by each parent are then written along the edges of the square, and the combinations of gametes are filled in the boxes of the square. The resulting genotypes and phenotypes of the offspring can be determined by counting the number of each type of genotype or phenotype.

The phenotypic ratio of F2 generation in a dihybrid cross can be summarized as follows:

- If both traits are completely dominant, the phenotypic ratio will be 9:3:3:1.
- If one trait is completely dominant and the other is recessive, the phenotypic ratio will be 3:1.
- If both traits exhibit incomplete dominance, the phenotypic ratio will be 1:2:1.
- If both traits are codominant, the phenotypic ratio will be 1:2:1.

Therefore, the correct answer is option D, which represents the phenotypic ratio of 9:3:3:1 in a dihybrid cross where both traits are completely dominant. This ratio is obtained when the F1 generation is crossed with each other or with their homozygous parents. The ratio indicates that 9 out of 16 (56.25%) offspring will exhibit both dominant traits, 3 out of 16 (18.75%) will exhibit one dominant and one recessive trait, 3 out of 16 (18.75%) will exhibit the other dominant and the other recessive trait, and 1 out of 16 (6.25%) will exhibit both recessive traits.