Four successive members of the first series of transition metals are listed below. For which one of the of standard potential `(E_(M^(2+)//M)^(@))` value has a positive sign ?

Options

A. `Ni(Z=28)`
B. `Cu(Z=29)`
C. `Fe(Z=26)`
D. `Co(Z=27)`

Explanation

To determine which of the four successive members of the first series of transition metals has a positive standard electrode potential `(E_(M^(2+)//M)^(@))`, we need to analyze the given metals: Nickel (Ni), Copper (Cu), Iron (Fe), and Cobalt (Co).

### Step-by-Step Solution:

1. **Identify the Metals and Their Electrode Potentials**:
- Nickel (Ni): `E = -0.25 V`
- Copper (Cu): `E = +0.34 V`
- Iron (Fe): `E = -0.44 V`
- Cobalt (Co): `E = -0.28 V`

2. **Compare the Electrode Potentials**:
- We need to look for the metal with a positive standard electrode potential.
- The values are:
- Ni: `-0.25 V`
- Cu: `+0.34 V`
- Fe: `-0.44 V`
- Co: `-0.28 V`

3. **Identify the Positive Value**:
- Among the listed values, only Copper (Cu) has a positive standard potential of `+0.34 V`.

4. **Conclusion**:
- Therefore, the metal for which the standard potential `(E_(M^(2+)//M)^(@))` has a positive sign is **Copper (Cu)**.

### Final Answer:
The correct option is **B. Copper**.

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