Which among the following served as Chief Minister for the longest term?

Hukum Singh
Pt. Bhagwan Dayal
Bansi Lal
Rao Birender Singh

The correct answer is Bansi Lal.

Bansi Lal served as the Chief Minister of Haryana for a total of 6 years and 10 months, from 1977 to 1979, from 1985 to 1987, and from 1991 to 1996.

Hukum Singh served as the Chief Minister of Haryana for a total of 3 years and 10 months, from 1967 to 1970.

Pt. Bhagwan Dayal served as the Chief Minister of Haryana for a total of 1 year and 10 months, from 1970 to 1971.

Rao Birender Singh served as the Chief Minister of Haryana for a total of 1 year and 10 months, from 1996 to 1999.