Step 1: Write the balanced chemical equation.
The given reaction is
\[
2\mathrm{N_2O_5}(g)\rightarrow 4\mathrm{NO_2}(g)+\mathrm{O_2}(g)
\]
Step 2: Write the rate relation.
For the reaction,
\[
2\mathrm{N_2O_5}\rightarrow 4\mathrm{NO_2}+\mathrm{O_2}
\]
the rate relation is
\[
-\frac{1}{2}\frac{d[\mathrm{N_2O_5}]}{dt}
=
\frac{1}{4}\frac{d[\mathrm{NO_2}]}{dt}
\]
Step 3: Substitute the given rate of disappearance of \(\mathrm{N_2O_5}\).
Given,
\[
-\frac{d[\mathrm{N_2O_5}]}{dt}
=
1.2\times10^{-5}\ \mathrm{mol\ L^{-1}\ s^{-1}}
\]
Using the stoichiometric ratio,
\[
\frac{d[\mathrm{NO_2}]}{dt}
=
\frac{4}{2}
\left(
-\frac{d[\mathrm{N_2O_5}]}{dt}
\right)
\]
\[
\frac{d[\mathrm{NO_2}]}{dt}
=
2\times 1.2\times10^{-5}
\]
\[
\frac{d[\mathrm{NO_2}]}{dt}
=
2.4\times10^{-5}\ \mathrm{mol\ L^{-1}\ s^{-1}}
\]
Step 4: Final conclusion.
Therefore, the rate of production of \(\mathrm{NO_2}\) is
\[
\boxed{2.4\times10^{-5}\ \mathrm{mol\ L^{-1}\ s^{-1}}}
\]
Hence, the correct option is
\[
\boxed{(3)}
\]