Here's why:

* Codons are three-nucleotide sequences that specify a particular amino acid during protein synthesis.

* mRNA carries the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm, where protein synthesis occurs.

* Ribosomal RNA (rRNA) forms the structural and catalytic core of ribosomes.

* Transfer RNA (tRNA) carries specific amino acids to the ribosomes, matching them to the codons on mRNA.

So, while all three types of RNA play crucial roles in protein synthesis, only mRNA contains the codons that dictate the amino acid sequence of the protein being built.