Perhaps the closest that the Beatles ever came to a reunion while all were still alive was when they all appeared on self-titled album from 1973, though not all on the same track.

Ringo's

Explanation:

In 1973, the Beatles (Lennon, McCartney and Harrison) all collaborated with Ringo Starr on their album Ringo. It was three years after the group split, and they met again for a 'reunion'. This is the correct answer because in 1973 Ringo was the only person to have a self-titled album.