Which behavioral psychology retail technique did the UK government's 'Behavioral Insights Team' famously use to increase tax compliance by 15% in 2011-2012?

Government agencies have increasingly adopted retail consumer psychology principles to influence public behavior in positive ways. This approach, often called 'nudge theory,' uses subtle psychological cues rather than mandates to guide citizens toward better choices. How well do you understand these behavioral science applications in government communications and public policy implementation?

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Social proof messaging (stating that most people in their area had already paid their taxes)
Scarcity principle (warning that tax benefits would expire soon)
Authority principle (featuring signatures of high-ranking officials)
Reciprocity principle (offering small incentives for early payment)