Apache SpamAssassin is one of the most widely used open-source tools for spam email detection.

It applies a rule-based system combined with Bayesian filtering, heuristics, and collaborative filtering methods to classify incoming emails as spam or legitimate.

Option A (Spam emails): Correct, this is the main function.

Option B (Big attachments): Incorrect. Large attachment filtering is not its primary purpose.

Option C (Email with virus): Incorrect. That falls under antivirus or malware detection tools, not SpamAssassin.

Option D: Incorrect since A is valid.

Thus, the correct answer is Option A (Spam emails).