Which language learning product's retail strategy first popularized the 'spaced repetition system' (SRS) as a core selling point, revolutionizing how vocabulary retention was marketed to consumers?

In the world of linguistics and language products, branding strategies have evolved dramatically over time. Dictionary publishers, language learning platforms, and translation services have all employed innovative retail strategies to differentiate themselves in the market. This trivia question explores a pivotal moment in linguistic product history that revolutionized how language learning tools were marketed to consumers worldwide.

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Rosetta Stone's immersion method (1992)
SuperMemo's algorithm-based learning system (1987)
Duolingo's gamified language platform (2011)
Pimsleur's audio-based learning program (1963)