What consumer psychology principle did Duolingo most effectively leverage to disrupt the language learning market and build brand loyalty?

In the competitive language learning industry, certain brands have revolutionized how consumers engage with linguistic education. Product positioning and brand psychology have transformed what was once dominated by textbooks and classroom settings into diverse digital experiences. This poll tests your knowledge of how one major player disrupted the market through innovative consumer psychology and brand positioning.

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Loss aversion through the 'streak' feature that makes users fear missing daily lessons
Sensory marketing by incorporating multiple language accents in their audio components
Scarcity marketing by limiting free users to only certain languages
Authority principle by highlighting endorsements from linguistic professors