Why is الْمُسْلِمِينَ the maf'ul bihi in نَصَرَ الْمُسْلِمِينَ?

Maf'ul Bihi: The Receiver of the Action

• Because the Muslims performed the helping.
• Because the Muslims received the help and the fa'il is understood from the verb.
• Because every plural noun is an object.
• Because it follows a preposition.