A YouTube clip showing that it IS Tango: https://youtu.be/AzHyXhTlT90
A summary of the film here: https://en.wikipedia.org/wiki/The_Conformist_%28film%29
This is a much-reproduced still taken form the film. This image is black and white while the film itself is in colour
I find this image interesting because it has so many layers. I have not watched the film (and would love to hear from those who have) but as I understand it, the dance is intended to show a loving alternative to the conformism demand of people by the fascist regime. Why was such an idea attractive to Bertolucci in late 1960s post-war Italy? The dancing is perfunctory, but an unusual example of a women couple dancing tango on film.

Image URL: https://luigilocatelli.files.wordpress.com/2012/03/936full-the-conformist-screenshot.jpg