What key observations should a seller-server make concerning customers to ensure responsible alcohol service?

The key observations that a seller-server should make concerning customers to ensure responsible alcohol service are: How they look, What they do, How they react, and How much alcohol has been purchased or consumed. These observations are crucial in assessing a customer's demeanor, behavior, and alcohol consumption levels to help prevent over-serving and promote a safe drinking environment.

The other options are distractions and not relevant to responsible alcohol service. Choices A, C, and D focus on personal preferences and details that do not impact the responsible service of alcohol. It is essential for seller-servers to pay attention to cues that directly relate to the customer's alcohol consumption and behavior to fulfill their responsibility in serving alcohol responsibly.