Final answer:
Most people have five fingers because the polydactyl allele is rare in the human population and certain genetic elements may suppress its expression. Surgical removal of the sixth finger can also occur after birth.
Step-by-step explanation:
Most people have five fingers because the allele encoding polydactyly (six fingers) is a rare trait in the human population (d). While the allele is dominant, it is not present in high enough frequency to result in most individuals having six fingers. Additionally, certain genetic elements may suppress the expression of the polydactyl gene (a). In cases where individuals are born with six fingers, surgical removal of the sixth finger can also occur after birth (c).