Final answer:
The counter melody to "ob-la-di, ob-la-da" is played by the guitar and piano.
Step-by-step explanation:
The counter melody to "ob-la-di, ob-la-da" is played by the guitar and piano. The guitar typically plays a catchy riff that follows the main melody, while the piano adds harmonies and fills in the gaps. These instruments work together to create a contrasting and complementary melody to the main theme of the song.