Answer: mental set
Explanation: Problem solving could simply be defined as a set of processes required to proffer solution or find answers to a problem. The most commonly identified barriers to problem solving include ; confirmation bias, mental set, functional fixedness, unnecessary constraints, and irrelevant information.
In the scenario above, the barrier to problem solving the individual is failing to overcome is the mental set, which could be explained when an individual is rigid on a particular method or procedure adopted in solving previous problems. There is every possibility that, the individual was successfully able to open previous doors by pushing the door and has become inclined to the idea of pushing on all doors to gain entry.