Network Behavior Analysis (NBA) tools are the best network defense against unknown types of attacks or stealth attacks in progress. NBA tools are devices or software that monitor and analyze the network traffic and activities, and detect any anomalies or deviations from the normal or expected behavior. NBA tools use various techniques, such as statistical analysis, machine learning, artificial intelligence, or heuristics, to establish a baseline of the network behavior, and to identify any outliers or indicators of compromise. NBA tools can provide several benefits, such as:

Detecting unknown types of attacks or stealth attacks that are not signature-based or rule-based, and that can evade or bypass other network defenses, such as firewalls, IDS, or IPS.

Detecting advanced persistent threats (APTs) that are low and slow, and that can remain undetected for a long time, by correlating and aggregating the network events and data over time and across different sources.

Detecting insider threats or compromised hosts that are authorized and trusted, but that exhibit malicious or suspicious behavior, by profiling and classifying the network entities and their interactions.

Providing early warning and alerting of the potential or ongoing attacks, and facilitating the investigation and response of the incidents, by providing rich and contextual information about the network behavior and the attack vectors.

The other options are not the best network defense against unknown types of attacks or stealth attacks in progress, but rather network defenses that have other limitations or drawbacks. Intrusion Prevention Systems (IPS) are devices or software that monitor and block the network traffic and activities that match the predefined signatures or rules of known attacks. IPS can provide a proactive and preventive layer of security, but they cannot detect or stop unknown types of attacks or stealth attacks that do not match any signatures or rules, or that can evade or disable the IPS. Intrusion Detection Systems (IDS) are devices or software that monitor and alert the network traffic and activities that match the predefined signatures or rules of known attacks. IDS can provide a reactive and detective layer of security, but they cannot detect or alert unknown types of attacks or stealth attacks that do not match any signatures or rules, or that can evade or disable the IDS. Stateful firewalls are devices or software that filter and control the network traffic and activities based on the state and context of the network sessions, such as the source and destination IP addresses, port numbers, protocol types, and sequence numbers. Stateful firewalls can provide a granular and dynamic layer of security, but they cannot filter or control unknown types of attacks or stealth attacks that use valid or spoofed network sessions, or that can exploit or bypass the firewall rules.