Mark David Chapman, 45, who shot and killed John Lennon 20 years ago, was denied parole yesterday after a 50-minute hearing at New York’s Attica state prison. Lennon’s widow, Yoko Ono, made a personal plea in writing to the New York State Parole Board, requesting that her husband’s assassin be kept behind bars. She said that she and Lennon’s children would live in fear if he were set free and that the killer might also be in danger if released. “Myself and John’s two sons would not feel safe for the rest of our lives,” she wrote. A Parole Board spokeswoman told The New York Times that none of Chapman’s or Lennon’s relatives were allowed to attend the hearing — which was Chapman’s first bid for parole — though Ono was permitted to state her position. Ono was in Japan yesterday for the opening of a John Lennon Museum, but her spokesman told The Times after Chapman’s hearing, “She respects the board’s decision.”