Step 1: Concept The difference in sound intensity levels in decibels (dB) is calculated as $L = 10 \log_{10}(I_{1}/I_{2})$.
Step 2: Meaning Attenuation by 20 dB means the difference between the initial and final level is 20.
Step 3: Analysis $20 = 10 \log_{10}(I_{initial}/I_{final}) \implies 2 = \log_{10}(I_{initial}/I_{final})$.
Step 4: Conclusion $I_{initial}/I_{final} = 10^{2} = 100$. The intensity decreases by a factor of 100.
Final Answer: (B)