Answer
a) This redox reaction does not occur spontaneously in the forward direction.
b) This redox reaction occurs spontaneously in the forward direction.
c)This redox reaction occurs spontaneously in the forward direction.
d) This redox reaction does not occur spontaneously in the forward direction.
Work Step by Step
Metal Activity series:
$Li\gt K\gt Ca\gt Na \gt Mg\gt Al\gt Mn\gt Zn\gt Cr\gt Fe\gt Ni\gt Sn\gt Pb\gt H_{2}\gt Cu\gt Ag\gt Au$
Activity decreases from left to right. More active metal will be oxidized if the reaction occurs spontaneously.
Now, see each reaction.
a) From Metal activity series we can see that 'Zn' is more active than 'Ni'. So, Zn will be oxidized into $Zn^{2+}$. On the other hand, Ni is less active so $Ni^{2+}$ will be reduced to Ni. But in the reaction we see that $Zn^{2+}$ is reduced to Zn and Ni is oxidized to $Ni^{2+}$. So, this reaction does not occur spontaneously in the forward direction.
b) From metal activity series it is seen that Ni is more active than Pb. So, Ni will oxidize to $Ni_{2+}$ and $Pb_{2+}$ will reduce to Pb. So, the written reaction occurs spontaneously in the forward reaction.
c) Al is more active than Ag in the metal activity series. So, Al will oxidize and $Pb^{2+}$ will reduce which can be seen in the reaction. So, this reaction will occur spontaneously in the forward direction.
d) Mn is more active than Pb in the metal activity series. So, Mn will oxidize. But here in the reaction it is reduced. So, the reaction does not occur spontaneously in the forward direction.
