The exhibited playbook requires intervention, which means that the playbook has reached a point where it needs a human operator to take action. The next step should be to go to the Incident Response tasks dashboard and run the pending actions. This will allow you to see the pending actions that need to be taken and to take those actions.
The other options are not correct. Option B will only show you the notification icon, but it will not allow you to run the pending input action. Option C will run the Mark Drive by Download playbook action, but this is not the correct action to take in this case. Option D is not a valid option.
Here are some additional details about pending actions in FortiSOAR:
Pending actions are actions that need to be taken by a human operator.
Pending actions are displayed in the Incident Response tasks dashboard.
Pending actions can be run by clicking on the action in the dashboard.

Option B: Adding more web servers to the real server pool will increase the overall capacity of the load balancer, which should help to resolve the issue of users not being able to access the website.
Option D: Adding a connection-pool to the FortiADC virtual server will allow the load balancer to cache connections to the web servers, which can help to improve performance and reduce the number of dropped connections.
Option A: Changing the persistence rule to LB_PERSIS_SSL_SESSJD would only be necessary if the current persistence rule is not working properly. In this case, the CPU usage on the FortiADC and the web servers is low, so the persistence rule is likely not the issue.
Option C: Disabling SSL between the FortiADC and the web servers would reduce the load on the FortiADC, but it would also make the website less secure. Since the bandwidth utilization is under 30%, it is unlikely that disabling SSL would resolve the issue.

SD-WAN is a feature that allows users to optimize network performance and reliability by using multiple WAN links and applying rules based on various criteria, such as latency, jitter, packet loss, etc. One way to ensure that the FortiGate DNS resolution times are as low as possible with the least amount of work is to configure local out traffic to use the outgoing interface based on SD-WAN rules with the interface IP and configure an SD-WAN rule to the DNS server. This means that the FortiGate will use the best WAN link available to send DNS queries to the DNS server according to the SD-WAN rule, and use its own interface IP as the source address. This avoids NAT issues and ensures optimal DNS performance. References: https://docs.fortinet.com/document/fortigate/7.0.0/sd-wan/19662/sd-wan

Using multiple VPN tunnels and BGP sessions for each internal segment is not scalable and efficient, especially when the number of segments, DCs or internet links per DC increases. A better solution is to use a single VPN tunnel per branch and segment traffic using virtual routing and forwarding (VRF) instances on BGP. This way, each VRF can have its own routing table and BGP session, while sharing the same VPN tunnel. Reference: https://docs.fortinet.com/document/fortigate/6.4.0/cookbook/103439/sd-wan-with-vrf-and-bgp