Step 1: Calculate the volume occupied by one mole of gas.
Given diffusion rate,
\[
0.082\,\mathrm{L\,s^{-1}}.
\]
In \(100\,\mathrm{s}\),
\[
V
=
0.082\times100
=
8.2\,\mathrm{L}.
\]
Step 2: Apply the ideal gas equation.
Using
\[
PV=nRT,
\]
where
\[
n=1,\qquad
T=300\,\mathrm{K},
\]
\[
P
=
\frac{nRT}{V}
=
\frac{1\times0.082\times300}{8.2}
=
3\,\mathrm{atm}.
\]
Hence,
\[
\boxed{3\,\mathrm{atm}}
\]
Therefore,
\[
\boxed{(B)}
\]
is the correct answer.