Step 1: Use the internal energy formula for diatomic gases.
Nitrogen (\(\mathrm{N_2}\)), oxygen (\(\mathrm{O_2}\)), and hydrogen (\(\mathrm{H_2}\)) are all diatomic gases.
At ordinary temperatures,
\[
U=\frac{5}{2}nRT.
\]
Step 2: Calculate the total number of moles.
\[
n=6+4+2=12.
\]
Step 3: Compute the total internal energy.
\[
U
=
\frac{5}{2}(12)RT
=
30RT.
\]
Hence,
\[
\boxed{30RT}
\]
Therefore,
\[
\boxed{(C)}
\]
is the correct answer.