Final answer:
The advertising slogan 'It floats' from 1891 refers to Ivory soap, which was known for its distinctive floating property due to air being blown through molten soap during production.
Step-by-step explanation:
The soap whose advertising slogan became "It floats" in 1891 is Ivory. This slogan emphasized the unique selling point of Ivory soap, which was its ability to float in water. This characteristic was the result of air being blown through the molten soap during the manufacturing process, making the soap less dense than water. The floating property of Ivory soap became a key part of its marketing and brand identity, differentiating it from other soaps available at the time.