Why does a pea plant with the genotype Rr produce some gametes that have the allele for round seeds?

The correct answer is - due to the law of segregation.

Explanation:

In the given case the genotype of Rr and as we know R is a dominant characteristic for round seeds and r represents the recessive allele for wrinkled seeds in a pea plant. So if a plant has Rr in its genotype it means it is a phenotypically round pea plant due to dominance in this heterozygous case.

The law of segregation states that two alleles of a gene of a specific trait will be distributed randomly and there is an equal chance of each allele to end up in the gametes, similarly in this case both alleles can be segregated to gametes and it is a random process.