Each number above molecule b refers to a codon. if an rna codon was uag, what would the corresponding dna codon be?

The corresponding DNA codon would be ATC.

The complementary DNA nucleotide for a:

RNA uracil is the adenine;

RNA adenine is the thymine;

RNA guanine is the cytosine.

Because a RNA strand is made from complementation of a DNA model-strand, when we have a RNA codon, the DNA codon that was in its origin is simply the combination of the three DNA nucleotides that are complementary to the RNA codon.