(i) The most reactive non-metal is (B) because its electron gain enthalpy is most negative. (ii) The most reactive metal is (A) because it has very low value of first ionisation energy. (iii) The least reactive element is (D) as it has highest value of first ionisation energy and positive electron gain enthalpy. (iv) The metal forming binary halide is (C) because a metal that forms a stable binary halide will be an alkaline earth metal of second group. Its I2 will not be much higher than IE1.