Which word best describes someone who is watchful and alert?

The term "vigilant" is the most appropriate descriptor for someone who is watchful and alert. To be vigilant implies a state of heightened awareness and attentiveness to potential dangers or issues, reflecting a proactive stance in observing the environment. It suggests that the individual is not only aware but is actively monitoring their surroundings to respond swiftly if necessary.

In contrast, while "attentive" can also imply a level of awareness, it is more about being focused on a specific task or person rather than maintaining a broad alertness to potential threats or changes in the environment. "Inattentive" clearly denotes the opposite of being watchful, indicating a lack of focus or awareness, and "careless" suggests negligence and a disregard for one’s surroundings, which is also contrary to the concept of being watchful and alert.