While all choices are correct meanings, IPS in the security field stands for Intrusion Prevention System. Intrusion prevention systems are considered extensions of intrusion detection systems because they both monitor network traffic and/or system activities for malicious activity.
** answer explanation is wrong. IPS are an evolution of IDS, an IDS will tell you there is an attack but won't do anything about it, alert only as they tend not to sit inline if network traffic. IPS sit inline and as the P denotes when an attack occurs the device can be configured to actually stop/block the traffic rather than just alert **