C

There are four dorsal interosseous muscles in the hand.  
• The 1st acts on the index finger, the 4th on the ring finger, while the 2nd inserts on the radial side of the middle finger and the 3rd on its ulnar side, giving the middle finger two separate dorsal interossei [Eladoumikdachi, 2002, PMID 12360058].  
Hence the middle finger is the only digit with two dorsal interossei.