Which European power intervened in Bulgaria in 1877-78 to the benefit of Bulgarian nationalists seeking to gain their independence from the Ottoman Empire?

The correct answer is - Russia.

The Bulgarians tried to gain independence from the Ottoman Empire, and a key component in that was the Bulgarian National Revival, and in 1876 the Bulgarians had an uprising in the month of April, but the Ottomans were too strong so it was unsuccessful. This triggered Russia to intervene, since it had a big interest in the Balkans, so that prompted another war between the Russians and the Ottomans, with the Russians winning and Bulgaria managing to get its independence in 1878.