Configuring MAC Filtering will only allow approved MAC Addresses to connect to the router. This is the BEST option of those given, CompTIA questions are often worded like this. In reality the best option would be to change the WiFi password and verify the encryption settings first, but this was not an option.
In computer networking, MAC address filtering is a security access control method whereby the MAC address assigned to each network interface controller is used to determine access to the network.
MAC addresses are uniquely assigned to each card, so using MAC filtering on a network permits and denies network access to specific devices through the use of blacklists and whitelists. While the restriction of network access through the use of lists is straightforward, an individual person is not identified by a MAC address, rather a device only, so an authorized person will need to have a whitelist entry for each device that they would like to access the network.
While giving a network some additional protection, MAC filtering can be circumvented by using a packet analyzer to find a valid MAC and then using MAC spoofing to access the network using that address. MAC address filtering can be considered as security through obscurity because the effectiveness is based on "the secrecy of the implementation or its components".