Such artists as édouard manet, edgar degas, and henri de toulouse-lautrec had previously explored the subject on which pablo picasso's les demoiselles d'avignon is focused. what is this subject?

I would contend that the right answer is prostitution.

Explanation: Just to elaborate a little bit on the answer, it can be added that those three artists—all French, since prostitution became a subject of great interest in France and other parts of Europe in the nineteenth-century—explored the traditional theme of prostitution, although with a different approach: a very contemporary one, which showed the crude reality of it openly and without reservations. Manet's Olympia (1865) caused a great scandal when he exhibited it at the Paris Salon, in spite of its references to classical models, such as Titian's Venus of Urbino. He portrayed a prostitute that looked at the viewer without any shyness. In addition, both Degas and Toulouse-Lautrec painted brothels. Examples of this are The Madam's Name Day (1876), by Degas, and Salon de la Rue des Moulins (1894), by Toulouse-Lautrec.