Final answer:
Most people have five fingers even if the allele for polydactyly is dominant because the allele for six fingers is very rare in the human population.
Step-by-step explanation:
If the allele encoding polydactyly (six fingers) is dominant, most people have five fingers because the polydactyl allele is very rare in the human population. This is not the result of embryonic lethality or suppression by genetic elements, nor is the sixth finger commonly removed after birth. The rarity is due to the fact that despite being a dominant allele, it is not widespread in the population, and thus most people inherit the more common allele for five fingers.
The reason why most people have five fingers despite the dominant allele encoding polydactyly is because polydactyly is a rare mutation (option a). This means that the allele responsible for polydactyly is not widespread in the human population (option d). Although the allele is dominant, it is not common for individuals to inherit or express the allele that results in six fingers.