Concept:Hadith 18 of an-Nawawi focuses on piety, righteous deeds, and good behaviour toward people.
Explanation:This hadith is reported from Abu Dharr and Mu'adh ibn Jabal (may Allah be pleased with them).
The Prophet (peace be upon him) said: "Fear Allah wherever you are."
This means a believer must show piety (taqwa) in every situation, whether open or secret.
He also said: "Follow up a bad deed with a good deed; it will wipe it out."
This teaches that a believer should work righteously so that good deeds erase the effect of sins.
The Prophet further advised believers to behave well towards people.
Therefore, the main message of Hadith 18 is that believers should have piety and perform righteous deeds.
The other options do not reflect the core teaching of this hadith.
Answer:C. Have piety and work righteously