Final answer:
When a stop codon is present in the A site, release factors bind to this site to terminate protein synthesis, leading to the dissociation of the ribosomal subunits and release of the new protein.
Step-by-step explanation:
When a stop codon is in the A site of a ribosome, the correct event is that release factors bind to the A site. These release factors are responsible for signaling the end of the protein synthesis process. They instruct the ribosomal enzyme peptidyl transferase to add a water molecule to the carboxyl end of the last amino acid. This event leads to the release of the newly synthesized polypeptide chain from the tRNA in the P site, after which the small and large ribosomal subunits and other components dissociate and can be reused for another round of protein synthesis.