Explanation:

NABARD (National Bank for Agriculture and Rural Development) is India's apex development bank, primarily responsible for providing and regulating credit and other facilities for the promotion and development of agriculture, small-scale industries, cottage and village industries, handicrafts, and other rural crafts. Its main function, as stated in option (2), is 'Refinancing to agricultural financing institutions'. This means NABARD provides financial assistance to commercial banks, regional rural banks, and cooperative banks so they can, in turn, lend to farmers and rural enterprises. Option (1) 'Agricultural research' is primarily conducted by ICAR (Indian Council of Agricultural Research). Option (3) 'Farmer loaning' is done by commercial banks and other financial institutions, with NABARD providing the refinance. Option (4) 'Providing fertilizers' is the role of fertilizer companies and government subsidies, not NABARD's direct function.