Which response best characterizes a comatose patient?

A comatose patient is best characterized by the inability to respond to external stimuli, which includes remaining unarousable with eyes closed. In this state, a person does not exhibit voluntary movements or purposeful responses to the environment. This lack of consciousness means that they do not experience awareness, and they cannot be awakened or respond to sound or physical touch.

The other options illustrate varying degrees of arousal and responsiveness, which are not applicable to a patient in a coma. Responding to loud stimuli or opening eyes to tactile stimulus indicates some level of awareness or responsiveness, while engaging in slow verbal responses suggests a degree of cognitive function that a comatose patient does not possess. Therefore, the defining characteristic of a comatose patient is the persistent unresponsiveness and closed eyes that signify a severe impairment of consciousness.