Which of the following factors is critical in ensuring the success of sealant placement?

Proper tooth isolation is critical in ensuring the success of sealant placement because it prevents saliva and other contaminants from interfering with the adhesion of the sealant to the tooth surface. Effective isolation helps create a clean, dry environment, which is essential for the bonding process. Without adequate isolation, moisture can compromise the sealant's ability to adhere, potentially leading to failures such as leakage or loss of the sealant over time.

The other factors, while they contribute to the overall procedure, do not play as pivotal a role in the initial success of the sealant itself. For instance, the choice of sealant material can affect durability but is less critical than ensuring that the tooth surface is correctly isolated prior to application. Applying sealants with gloves is more about maintaining hygiene than influencing the sealant's functional success, and using a high-speed handpiece is irrelevant to the adhesion process in this context. Thus, proper tooth isolation stands out as the fundamental practice that directly impacts the effectiveness of sealant placement.