Which electrode is used to measure the electrode potential of any electrode?

If the reduction potential is more,then

The standard reduction potentials at $298 \ K$ for the following half-reactions are given:
$Zn^{2+}_{(aq)} + 2e^- \rightleftharpoons Zn_{(s)}; E^\circ = -0.762 \ V$
$Cr^{3+}_{(aq)} + 3e^- \rightleftharpoons Cr_{(s)}; E^\circ = -0.740 \ V$
$2H^+_{(aq)} + 2e^- \rightleftharpoons H_{2(g)}; E^\circ = 0.00 \ V$
$Fe^{3+}_{(aq)} + e^- \rightleftharpoons Fe^{2+}_{(aq)}; E^\circ = 0.770 \ V$
Which is the strongest reducing agent?

The standard electrode potentials for some half-reactions are given below. Which statement is correct?
$(A) Sn^{4+} + 2e^{-} \rightarrow Sn^{2+}, E^o = +0.15 \ V$
$(B) 2Hg^{2+} + 2e^{-} \rightarrow Hg_{2}^{2+}, E^o = +0.92 \ V$
$(C) PbO_2 + 4H^{+} + 2e^{-} \rightarrow Pb^{2+} + 2H_2O, E^o = +1.45 \ V$

Given:
$E^o_{Fe^{3+} /Fe} = -0.036 \ V, E^o_{Fe^{2+} /Fe} = -0.439 \ V$
The value of standard electrode potential for the change,
$Fe^{3+}_{(aq)} + e^- \rightarrow Fe^{2+}_{(aq)}$ will be ........ $V$.

For $Mg^{2+} + 2e^{-} \rightarrow Mg_{(s)}$,$E^{0} = -2.37 \, V$ and for $Cu^{2+} + 2e^{-} \rightarrow Cu_{(s)}$,$E^{0} = +0.33 \, V$. The $emf$ of the cell $(E^{0}_{Cell})$ is .......... $V$.