Step 1: Use the sound level difference formula.
\[
\Delta L
=
10\log_{10}\left(\frac{I_2}{I_1}\right)
\]
Step 2: Substitute the given values.
\[
\Delta L
=
10\log_{10}\left(\frac{10^{-4}}{10^{-6}}\right)
=
10\log_{10}(10^2)
=
10\times2
=
20\,\mathrm{dB}
\]
Hence,
\[
\boxed{20\,\mathrm{dB}}
\]
Therefore,
\[
\boxed{(B)}
\]
is the correct answer.