The problem presents a partial Punnett square with two offspring genotypes, both $Rr$. We are asked to determine the genotypes of the parent plants that could produce this result.

1. Problem Description

The problem presents a partial Punnett square with two offspring genotypes, both $Rr$. We are asked to determine the genotypes of the parent plants that could produce this result.

2. Solution Steps

A Punnett square shows the possible genotypes of offspring resulting from a cross between two parents. Each parent contributes one allele to the offspring's genotype. We are given that two offspring have the genotype $Rr$. This means that each offspring received one $R$ allele from one parent and one $r$ allele from the other parent. We need to consider possible parent genotypes that could produce $Rr$ offspring.

Let's examine the possible parent crosses given as options.

A. $RR \times Rr$:

The possible offspring genotypes are $RR$ and $Rr$. This could produce $Rr$ offspring.

B. $RR \times rr$:

The only possible offspring genotype is $Rr$. This could also produce $Rr$ offspring. However, this cross would result in all offspring being $Rr$, but the provided Punnett Square does not necessarily represent all offspring.

C. $Rr \times rr$:

The possible offspring genotypes are $Rr$ and $rr$. This could produce $Rr$ offspring.

D. $rr \times rr$:

The only possible offspring genotype is $rr$. This cannot produce $Rr$ offspring.

We need to determine which of the remaining options A, B, or C is most likely. Considering we only see $Rr$ in the given part of the Punnett square, either cross involving $RR$ would have resulted in at least some $RR$ offspring. The fact we only see $Rr$ suggests that both parents must have at least one $r$ allele. Thus, $Rr \times rr$ is the best answer.

3. Final Answer

C. Rr x rr