NIDS stands for Network Intrusion Detection System. This system monitors the traffic on the network for any suspicious activity and sends alerts when such activity is detected. It's a passive system, meaning it won't take any action other than alerting the specified recipients.
The key point here is that NIDS does not affect the availability of the company's services. It does not block any traffic itself, even if it's malicious. This ensures that false positives (legitimate traffic that is incorrectly flagged as malicious) do not disrupt the company's services.
In contrast, a Network Intrusion Prevention System (NIPS), a Web Application Firewall (WAF), and a Reverse Proxy could potentially block traffic, which could lead to service disruption if there are false positives. Therefore, NIDS would be the best choice given the company's requirements.