For which film did Amitabh Bachchan won the National Award for best Actor?

Amitabh Bachchan won the National Film Award for Best Actor for his performance in the film "Paa," released in 2009. In this film, he portrayed a 12-year-old boy suffering from progeria, a rare genetic disorder that causes rapid aging. His compelling performance received widespread acclaim and highlighted his versatility as an actor.