23,463 I like 2,985 I do not like
According to multiple sources (examples linked below), Post-it Notes were test-marketed in four cities in 1977, with the national roll-out following in 1980.
Apart from the technicality that the product originally was sold under the name "Press & Peel" (which would be a stretch even if the category were "brands" instead of "inventions"), "C" was the correct answer. Therefore, the contestants should have lost only $80,000 (not $800,000) on the question.
December 1984 article: http://goo.gl/Kbhq4
December 2010 article: http://goo.gl/BLfXC
Official Facebook profile: http://goo.gl/5W5Yp