The structure which is missing from the diagram, but helps mRNA and tRNA bind together is the ribosome. Option A is correct.
Ribosomes make up a complex macromolecular machine, located within all living cells, which is the place where biological protein are synthesized. Ribosomes link amino acids together in the order specified by messenger RNA (mRNA) molecules. Ribosomes are made up of two fundamental components: the small ribosomal subunits, which read the mRNA, and the large subunits, which join amino acids to form a polypeptide chain.