Which of the following leaders was known as the 'Frontier Gandhi' for his…

Concept

Leaders of the freedom movement are widely remembered by honorific sobriquets, and such a title is normally built from two elements: a marker of the region or community the leader mobilised, and a marker of the political method or model the leader followed. When a title pairs a place-name with the name of another leader, it asserts that this person led that region's movement along the second leader's lines. A sobriquet of the form 'X Gandhi' therefore identifies whoever built a non-violent, Gandhian mass movement in the region named by X.

Applying it here

In this title 'Frontier' stands for the North-West Frontier Province of British India, the Pashtun-majority province beyond the Indus that is today Khyber Pakhtunkhwa in Pakistan. The leader who built a non-violent mass movement there was Khan Abdul Ghaffar Khan. In 1929 he founded the Khudai Khidmatgar, meaning 'Servants of God' and popularly called the Red Shirts, a disciplined volunteer body pledged to non-violence. It allied with the Indian National Congress, took part in the Civil Disobedience Movement, and campaigned steadily for Hindu-Muslim unity, and he opposed the partition of India on communal lines. Because he carried Gandhian satyagraha into the Frontier, he came to be called the 'Frontier Gandhi'; his own people called him Badshah Khan or Bacha Khan. India conferred the Bharat Ratna on him in 1987, the first person who was not an Indian citizen to receive that award.

Cross-check

Matching each leader with the title he is actually remembered by keeps the mapping straight:

Only the Frontier title is tied to the North-West Frontier Province, so the leader asked for in this question is Khan Abdul Ghaffar Khan.