Katihar had the highest proportion of Scheduled Tribe population among Bihar’s districts in the 2011 Census — option 1.

The setting. Bihar has a small tribal population by national standards — a little over 1 per cent of the state’s people, against about 8·6 per cent for India as a whole. The figure fell sharply in 2000, when the tribal-majority Chhotanagpur and Santhal Parganas divisions were separated to form Jharkhand. What remains is concentrated in two belts :

Why Katihar. The district lies on the Bihar-West Bengal-Jharkhand margin, and Santhal and Oraon families settled there in numbers during the nineteenth century, drawn by land reclamation and later by tea and jute cultivation in the neighbouring region. That history left Katihar with the largest tribal share of any district in the present state.

The four options are not random. All four are genuine districts of tribal concentration, and Purnia, Jamui and Banka would each be a reasonable guess. The question therefore rewards knowing not merely where the tribal population of Bihar lives, but which district stands at the top of the list.

Related census facts for Bihar in 2011 : the Scheduled Caste share is far larger at about 15·9 per cent, the state’s population was 10·41 crore, its density 1,106 persons per square kilometre — the highest of any Indian state — and its sex ratio 918.

Hence, the answer is Katihar.