Which Russian composer wrote 'Pictures at an Exhibition'?

The Russian composer who wrote Pictures at an Exhibition was Modest Mussorgsky. The work is a ten-movement piano suite inspired by artworks created by Mussorgsky’s friend Viktor Hartmann.

Hartmann died suddenly in 1873 at the age of 39. The critic Vladimir Stasov organized a memorial exhibition of more than 400 of Hartmann’s works in Saint Petersburg in 1874. Mussorgsky visited the exhibition and transformed selected images into music, completing the suite remarkably quickly from 2 to 22 June 1874.

The recurring “Promenade” theme represents the composer walking through the exhibition. Individual movements portray images such as “The Old Castle,” “The Ballet of the Unhatched Chicks,” and “The Great Gate of Kyiv.” Some of Hartmann’s original artworks have since been lost, so the music preserves impressions of them.

The suite became widely known through orchestrations, especially Maurice Ravel’s celebrated 1922 version. Rimsky-Korsakov did edit and publish a later version of the score, but he did not compose the work. That distinction is the most likely source of confusion in this question.