The correct option is D Ca and Cu
Standard reduction potentials (SRP) of the half cell electrodes are calculated and arranged in a series of increasing order which is known as electrochemical series.
The standard electrode potential of H+/H2 couple is zero.
Here, a negative E0 signifies that it has high tendency to get oxidised and the redox couple is a stronger reducing agent than the H+/H2 couple.
A positive E0 signifies that it has high tendency to get reduced and the redox couple is a stronger oxidising agent than the H+/H2 couple.
Electrochemical series helps in the selection of electrode assemblies to construct the galvanic cells of desired EMFs.
For a galvanic cell to provide high cell voltage, the half cell having higher reduction potential and lower reduction potential should be coupled.
In given electrochemical series,
Au+(aq)+e−→Au(s) (Highest value)
Ca2+(aq)+2e−→Ca(s) (Lowest value)
E0cell=SRP of substance reduced−SRP of substance oxidised
(SRP - Standard reduction potential)
E0cell=E0cathode (red)−E0anode (red)
For (a),
E0cell=1.69−(−0.44)
E0cell=2.13 V
For (b),
E0cell=1.69−(−0.13)
E0cell=1.82 V
For (c),
E0cell=1.69−0.34
E0cell=1.35 V
For (d),
E0cell=0.34−(−2.87)
E0cell=3.21 V
Thus, the SRP difference will be maximum for Ca and Cu.
Hence, it gives the maximum EMF for a voltaic cell.