Which word signifies a lack of response to external stimuli?

The chosen word, insensible, accurately signifies a lack of response to external stimuli. It refers specifically to being unable to perceive, feel, or respond to sensations or emotions, typically due to a physical or mental state. This word conveys a more profound absence of awareness or responsiveness than simple unresponsiveness, which may not necessarily entail a loss of feeling or sensation.

In contrast, the other options convey related but distinct meanings. Unresponsive suggests a failure to respond but does not imply a total lack of perception or feeling; it can be a temporary state. Imperturbable indicates a level of calmness and composure in the face of stimuli, suggesting a conscious control over reactions rather than an absence of response. Indifferent conveys a lack of interest or concern, which may imply awareness but a decision not to engage or respond. Thus, insensible is the most appropriate choice for describing a complete lack of response to external stimuli.