Step 1: Recall the function of a Draeger gas mask.
A Draeger gas mask uses chemical absorbents and catalysts to remove several harmful gases from inhaled air.
Step 2: Identify the gas that cannot be filtered.
The standard Draeger gas mask is not suitable for filtering
\[
\boxed{\text{Nitrous fumes (nitrogen oxides)}.}
\]
Hence,
\[
\boxed{(B)}
\]
is the correct answer.