Final answer:
After DNA replication in a somatic cell with 20 chromosomes, the number of chromosomes remains 20, but each now has two sister chromatids.
Step-by-step explanation:
If we start with a somatic cell with 20 chromosomes and it undergoes DNA replication, the number of chromosomes would remain the same, that is, 20. However, each chromosome would now consist of two sister chromatids, thus the total amount of DNA would have doubled. When chromosomes are counted, we typically count each centromere as one chromosome, so chromatids are not counted separately. After DNA replication, despite the increase in chromatids, the number of chromosomes as counted during cell division would still be 20. Therefore, the correct answer is D. 20.