The correct answer is "escapable shock".
Seligman and Maier's experiment was focused on the psychological phenomenon of learned helplessness. The first phase of the experiment included three groups of dogs who were put in harnesses. Dogs from groups 2 and 3, were tied in pairs and received electric shocks at random times. Dogs from group 2 were trained to press a lever to stop the electric shock. When dogs from group 3 pressed a lever, there was no effect on the electric shock. Therefore, dogs from group 3 perceived the electric shock as an inescapable painful stimulus. The second phase of the experiment involved a shuttle box. Dogs from all groups were put in these boxes and were free to jump outside the box when the electric shock was applied. However, dogs from group 3 (who were trained in phase 1 to believe that this was an inescapable shock) were passive and did not make any effort to escape the box. This was an experiment proving the theory of learned helplessness, according to which an animal or human who is chronically exposed to painful and inescapable stimuli, is unable to try and escape from new painful situations.