The BEST explanation of the role of DNA and RNA in making proteins is:
a
DNA unwinds into two separate strands, and a complementary copy of each is produced during replication. During transcription, a DNA strand is used to produce a complementary strand of mRNA, with uracil taking the place of thymine. During translation, each set of three bases in the mRNA results in an amino acid being added to a chain, and a protein is formed.
This option accurately describes the process of DNA replication, transcription to mRNA, and translation into proteins. It correctly identifies the substitution of uracil for thymine in RNA and explains how sets of three bases (codons) on the mRNA correspond to amino acids during protein synthesis. The other options have inaccuracies related to the processes involved.