basically, heartbeat is for connectivity and hello is for functionality

A, please check this https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-admin/high-availability/ha-concepts/ha-timers

A is correct

The hello interval es the interchange of HA information hello packets. Heartbeat Interval a a ICMP/Ping test.

A is correct.

It's A. It is visible clear in the shared links.

I will go for C.... because of the key "on the other HA firewall". While Hello is to each other.

This question was on the exam.. Nov 2023

Hello Interval - interval in milliseconds between hello packets that are sent to verify that the HA functionality on the other firewall is operational (range is 8,000 to 60,000; default is 8,000). from the 11.0 guide: https://docs.paloaltonetworks.com/pan-os/11-0/pan-os-admin/high-availability/ha-concepts/ha-timers#idcc1faa1c-5cd8-4830-90da-93e8effab23d

A. Keepalive is a ping. Heartbeat is a message sent to the other firewall to see if it's still functioning.

https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-admin/high-availability/ha-concepts/failover "Heartbeat Polling and Hello messages- The firewalls use hello message and heartbeats to verify that the peer firewall is responsive and operational. Hello messages are sent from one peer to the other at the configured Hello Interval to verify the state of the firewall. The heartbeat is an ICMP ping to the HA peer over the control link, and the peer responds to the ping to establish that the firewalls are connected and responsive. By default, the interval for the heartbeat is 1000 milliseconds. A ping is sent every 1000 milliseconds and if there are three consecutive heartbeat losses, a failovers occurs. For details on the HA timers that trigger a failover, see HA Timers." Since the failover here talks about pings, and Heartbeat is the one that does pings, I'm leaning towards Answer C.

Difference between Heartbeat and Hello: https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClUcCAK

A typo, option A is correct.

The question is a bit tricky but in this case option C is correct. https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClUcCAK

A is the way

A. Hello Interval (ms) Interval in milliseconds between hello packets that are sent to verify that the HA functionality on the other firewall is operational. The range is 8000-60000 ms with a default of 8000 ms for all platforms. https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-admin/high-availability/ha-concepts/ha-timers