Plant species has two alleles for leaf color. if plant species b has four alleles for leaf color , how will the two species differ in their appearance ?

Allele is the different version of the same gene. Mostly one gene contains two alleles for a trait for example gene for flower color have two alleles R (for red flower) and r(for white flower) so only these two color can be seen in phenotype.

So if a plant species have only two allele for leaf color it can express only either of two color in its leaf but if a species contains more than two alleles for leaf color that the leaf can express any of color out of them.

Therefore species B will differ in appearance because it can express four-color due to having four alleles for leaf color but other species having only two alleles can express only two color.  

the answer will be a) proteins i believe