The standard electrode potential (M³⁺/M²⁺) for V, Cr, Mn & Co are -0.26 V, - 0.41 V, + 1.57 V and +1.97 V, respectively. The metal ions which can liberate H₂ from a dilute acid are
(1) V²⁺ and Mn²⁺
(2) Cr²⁺ and CO²⁺
(3) V²⁺ and Cr²⁺
(4) Mn²⁺ and Co²⁺