Final answer:
Mussorgsky was part of 'The Mighty Handful', a progressive school of Russian composers.
Step-by-step explanation:
Mussorgsky was a member of a progressive school of 19th-century composers known as The Five or 'The Mighty Handful', and he was of Russian nationality. This group actively sought to produce a distinct Russian style of classical music during the post-Romantic era. Mussorgsky, alongside his contemporaries like Berlioz, Wagner, and later Stravinsky and Schoenberg, pushed the boundaries of music by either engaging in the nationalistic trends of the period or by expanding romantic musical techniques to their extremes.