The electrode potential of M²⁺/M of 3d-series elements shows positive value for :

Solution

Chemistry ma’am · 3 steps

Among the 3d metals, copper alone has a positive E°(M²⁺/M); the rest sit below hydrogen.

1. What the sign means. A negative E°(M²⁺/M) means the metal gives up electrons more easily than hydrogen and goes into solution as M²⁺. A positive value means the metal holds on to its electrons and is less reactive than hydrogen.

   Metal	E°(M²⁺/M) in V
   Fe	-0.44
   Co	-0.28
   Cu	+0.34
   Zn	-0.76

2. Why copper differs. Turning Cu(s) into Cu²⁺(aq) costs a lot of energy: copper has a high enthalpy of atomisation and a large second ionisation enthalpy. The hydration enthalpy of Cu²⁺ does not pay this back, so the overall change is unfavourable and E° comes out positive.
3. So. Only Cu has a positive M²⁺/M electrode potential. That is also why copper does not release H₂ from dilute acids. The answer is (c).

The idea to keep. The sign of E°(M²⁺/M) comes from atomisation plus ionisation enthalpy against hydration enthalpy. In the 3d series only Cu loses that balance, so only Cu is positive (+0.34 V).

Answer from the NTA final answer key. Solution by PrepWiser.