Concept:Hadith 11 of an-Nawawi teaches Muslims to choose what is certain over what is uncertain.Explanation:The Prophet (peace be upon him) advised believers to abandon matters that create doubt.Instead, a Muslim should hold firmly to what is clearly lawful and free from uncertainty.This guidance helps protect the heart and keeps one away from suspicious or unclear situations.Therefore, the main lesson is to avoid doubtful things in daily life.Answer:B. avoid doubtful things