Which of these vaccines must replicate to stimulate an immune response?

The choice indicating MMR as the correct answer is accurate because the MMR vaccine, which protects against measles, mumps, and rubella, consists of live attenuated viruses. For vaccines like MMR that contain live attenuated strains, the viruses need to replicate within the host's immune system to effectively stimulate a strong immune response. This replication allows the immune system to recognize the live viruses and respond appropriately, generating both antibody production and cellular immunity.

In contrast, the other vaccines mentioned do not require replication to generate an immune response. HPV9 is a recombinant vaccine that does not involve live active viruses, MCV4 is a conjugated vaccine that targets polysaccharide capsules without live organisms, and Tdap is an inactivated vaccine composed of toxins from bacteria, also lacking any live components. Therefore, the need for replication is a hallmark characteristic of live attenuated vaccines like MMR, making it the right choice in this context.