(1) republics that were part of the Soviet Union
Explanation:
The now defunct Soviet Union or Union of Soviet Socialist Republics (USSR) which is commonly and falsely referred to as just Russia comprised of 15 republics that were United under the Soviet banner.
The Estonian SSR, Belorussian SSR, and Moldavian SSR were some of those republics and indeed the 'SSR' you see in their names is an acronym for Soviet Socialist Republic i.e, Moldavian Soviet Socialist Republic.