Final answer:
The frequency of the recessive allele in the population is 0.04 or 4%.
Step-by-step explanation:
The frequency of the recessive allele in the population can be calculated using the formula: 2 * square root (q) where q represents the frequency of the recessive allele. In this case, the frequency of cystic fibrosis (ff) is 1 in 2,500 babies, which is equivalent to q = 1 / 2500 = 0.0004. Plugging this value into the formula, we get 2 * sqrt(0.0004) = 0.04. So, the frequency of the recessive allele in the population is 0.04 or 4% (option D).