The correct answer is: a) sitter
In the context of the statement, "the costume of the unidentified sitter suggests the approximate dating of this photo to 1890-1895," the word "sitter" refers to the person who is the subject of the photograph.
By examining the clothing or costume worn by the sitter, experts can make informed assessments about the historical period in which the photo was taken. Clothing styles, fashion trends, and accessories can provide valuable clues to help date old photographs.
Therefore, based on the clothing worn by the unidentified sitter, the photo is estimated to have been captured between 1890 and 1895, showcasing the fashion of that specific era.