Final answer:
A packet filter prevents unauthorized packets from entering the network by inspecting data packets against predefined security criteria and rules.
Step-by-step explanation:
A packet filter is responsible for controlling network access by monitoring outgoing and incoming packets and either allowing them to pass through or halting them based on the applied rule set. A packet filter functions by inspecting the data packets that travel across the network and determining whether they are allowed or denied based on the filtering rules. Therefore, the correct answer is A. Prevents unauthorized packets from entering the network.
Packet filters check the packets against a set of criteria such as source IP address, destination IP address, source port number, destination port number, and the protocol being used. This ensures that only authorized data is allowed to enter the network, enhancing security.
Contrary to the given reference information about blue filters which is related to light and color filtration and not relevant to network packet filtering, packet filters are solely concerned with the control of network traffic.