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.

Breacking the law and lazy