Answer: Positivity effect
Step-by-step explanation: In simple words, positivity effect refers to the situation when someone gets unexpected results from a research or analysis but the results stills happens to be positive and useful for future reference.
In the given case, the research group were expecting a negative review from the coal workers due to their continuous complaints about low pay and benefits. However the results came to be completely different as the workers were happy most of the time.
Hence the correct option is E.