Final answer:
To resolve an issue where a user is redirected from a search engine to blocked sites, running an anti-malware software scan is recommended to detect and remove potential malware causing the redirection.
Step-by-step explanation:
When a user is redirected to unwanted sites, particularly those blocked by a company's content filter system, this could indicate the presence of malware that has altered the behavior of the search engine results. Therefore, the best tool to resolve this issue would be A. Anti-malware software. This software is designed to detect, prevent, and remove malicious software and unwanted programs. Running a thorough scan using updated anti-malware software may help to identify and remove any malware that could be causing the redirection.