Brown seeds have this genotype Bb or BB. When we have the same allele, in this case BB, we have a homozygote. The question asks about a brown homozygote, that is BB, and a plant with white seeds, that is, bb. So, we have this cross, BB x bb. With this in mind, we are going to have either B or b gametes. In our Punnet square, we will have the following: 100% of heterozygotes Bb, phenotypcally this will mean that all seeds (100%) will be brown.