To determine the genotype of a black seed plant that could either be homozygous dominant ($\mathrm{BB}$) or heterozygous ($\mathrm{Bb}$), you need to perform a test cross. A test cross involves crossing the individual in question with a homozygous recessive individual. In this scenario, that would be a plant with white seed color, or genotype $\mathrm{bb}$.

A test cross is used because it can reveal whether the black seed plant carries the recessive $\mathrm{b}$ allele. When crossed with a homozygous recessive ($\mathrm{bb}$) plant:

• If the black seed plant is homozygous dominant ($\mathrm{BB}$), all offspring will inherit one $\mathrm{B}$ allele from the black seed parent and one $\mathrm{b}$ allele from the white seed parent, resulting in all offspring having black seeds ($\mathrm{Bb}$).
• If the black seed plant is heterozygous ($\mathrm{Bb}$), there is a 50% chance that an offspring will inherit the $\mathrm{B}$ allele and a 50% chance of inheriting the $\mathrm{b}$ allele from the black seed parent. This results in approximately half of the offspring having black seeds ($\mathrm{Bb}$) and half having white seeds ($\mathrm{bb}$).

By observing the seed colors of the offspring, you can determine whether the black seed plant was homozygous dominant or heterozygous. If any white seeds appear among the offspring, the black seed plant must be heterozygous ($\mathrm{Bb}$). If no white seeds appear, the black seed plant is likely homozygous dominant ($\mathrm{BB}$).

Therefore, the correct option for crossing to determine the genotype of the black seed plant is:

Option B $\mathrm{bb}$