Identify the cell commonly used in clocks

The cells commonly used in clocks are dry cells / alkaline cells.

In these cells, the cathode material is typically manganese dioxide, $\mathrm{MnO_2}$.

Determine the cathode reaction

At the cathode, reduction always occurs.

In the dry cell, manganese dioxide is reduced approximately as:

$\mathrm{MnO_2 + H_2O + e^- \rightarrow MnO(OH)}$

or equivalently, manganese changes from oxidation state $+4$ in $\mathrm{MnO_2}$ to $+3$ in the reduced product.

Check oxidation states of manganese

• In $\mathrm{MnO_2}$: $x + 2(-2) = 0 \Rightarrow x = +4$

• In the reduced form, manganese becomes approximately $+3$.

So the cathode process is:

$\mathrm{Mn^{+4} \rightarrow Mn^{+3}}$

which is a reduction.

Evaluate the options

• A: oxidation of $\mathrm{Mn}$ from $+2$ to $+7$
  Incorrect.
• B: reduction of $\mathrm{Mn}$ from $+4$ to $+3$
  Correct.
• C: oxidation of $\mathrm{Mn}$ from $+3$ to $+4$
  Incorrect.
• D: reduction of $\mathrm{Mn}$ from $+7$ to $+2$
  Incorrect.

Final answer

The reaction at cathode involves reduction of manganese from $+4$ to $+3$.