To solve this problem, we need to determine the genotype of a black seed plant by performing a test cross.$\newline$A test cross is used to determine whether an individual exhibiting a dominant trait is homozygous dominant (BB) or heterozygous (Bb).$\newline$The black seed color is dominant, represented by B, while the white seed color is recessive, represented by b.$\newline$To perform a test cross, we cross the black seed plant with a plant that is homozygous recessive (bb) for the trait.$\newline \bullet$If the black seed plant is homozygous dominant (BB), all offspring will have black seeds (Bb).$\newline \bullet$If the black seed plant is heterozygous (Bb), the offspring will have a 1:1 ratio of black (Bb) to white (bb) seeds.$\newline$Therefore, to determine the genotype of the black seed plant, we should cross it with a plant of genotype bb.$\newline$This corresponds to Option 4.