What is an early name for fixer that is still commonly used?

The term "Hypo" is a widely recognized early name for fixer in photographic processing, specifically referring to sodium thiosulfate. This compound is utilized in the development of photographic films and papers to remove unexposed silver halide crystals, effectively "fixing" the developed image. The casual name "Hypo" stems from its chemical name, and it has been adopted by photographers for decades, making it synonymous with the fixing process in photography.

In the context of the other terms, while hypochlorite and hydrogen peroxide are used in various chemical processes, they are not directly associated with photographic fixer. Acetic acid is commonly used as a stop bath in the developing process but does not serve the same function as fixer. Therefore, "Hypo" stands out as the correct answer due to its historical relevance and ongoing usage in the photography community.