Final answer:
The mutation rate per nucleotide is 0.001 when one mutation occurs in 1000 base pairs, making the correct answer A. 0.001 mutations per nucleotide.
Step-by-step explanation:
If one mutation occurs in 1000 base pairs (nucleotides) in a single generation, the per base pair (nucleotide) mutation rate is 1/1000. The correct calculation for the mutation rate per nucleotide is 1 divided by 1000, which equals 0.001 mutations per nucleotide. Therefore, the correct answer is A. 0.001 mutations per nucleotide.