A low-impact attack indicates that the host is not vulnerable to those attacks. A low-impact attack is an attack that does not exploit any known vulnerability on the target host or does not match any signature or anomaly rule on the FTD device5. A low-impact attack does not mean that the attack is not dangerous to the network or that the host is not within the administrator's environment. It simply means that the attack did not succeed in compromising or affecting the host.
The other options are incorrect because:
* All attacks are not listed as low until manually categorized. The FTD device automatically assigns an impact level to each attack based on various factors, such as vulnerability information, threat score, and confidence rating5. The impact level can be high, medium, or low, depending on how likely and how severe the attack is.
* The attacks are not necessarily harmless to the network. A low-impact attack may still cause some damage or disruption to the network, such as consuming bandwidth, generating noise, or distracting attention from other attacks6. A low-impact attack may also indicate that the attacker is probing or scanning the network for potential vulnerabilities or weaknesses7.
* The host is not necessarily outside the administrator's environment. A low-impact attack can target any host on the network, regardless of its location or ownership. A low-impact attack does not imply that the host is external or irrelevant to the administrator's environment.

Cisco warns that displaying captured packets directly on the screen can degrade system and network performance. The -w test option writes raw packets to a file instead of continuously parsing and displaying them on the terminal. The -s 1518 option sets the snapshot length to the default Ethernet maximum frame size so complete packets are captured, while -c 1000 automatically stops the capture after 1,000 packets. The BPF expression host 10.10.10.10 further reduces resource consumption by capturing only traffic involving the specified host. Options A, B, and C limit the captured traffic to varying degrees, but they omit -w and therefore continue sending decoded packet output to the screen. Consequently, Option D is the complete performance-conscious capture command. Cisco packet-capture procedure

Reference: https://www.cisco.com/c/en/us/support/docs/security/firepower-management-center/212699- configure-ftd-high-availability-on-firep.html