Why was Nelson Mandela sentenced to life imprisonment in 1964?

Nelson Mandela, leader of the African National Congress, an organization dedicated to protesting the apartheid policies of the South African government, was arrested in 1956 for treason, but  acquitted. On 12 June  1964, Nelson Mandela was sentenced to life in prison for undermining the apartheid government of South Africa, thereby avoiding the death penalty.

Hence, the correct option is (C) i.e, He dared to oppose apartheid regime in his country,