The reading frame of mRNA is:

a. the first three nucleotides of a gene.

b. any three nucleotides that can base pair with an anticodon.

c. a continuous, nonoverlapping set of three-nucleotide codons.

d. all of the nucleotides of an mRNA molecule.

a. the first three nucleotides of a gene.

Explanation:

In eukaryotes, the start codon is ATG, being, therefore, AGU in the messenger RNA (mRNA). This gene sequence is generally preceded by the 5' untranslated region (5' UTR). In the genetic code, this codon corresponds to the amino acid methionine