Metadata variables are custom fields that you can create on FortiManager to store additional information about objects or devices. They can be used as variables in Jinja2 CLI templates or scripts to apply configurations to multiple devices or objects. They do not apply only to non-firewall objects, but also to firewall objects such as addresses, services, policies, etc. The metadata format is not $<metadata_variable_name>, but @<metadata_variable_name>@. Reference := Using meta field variables, Metadata Variables are supported in Firewall Objects configuration, Technical Tip: New Meta Variables and their usage including Jinja Templates, Technical Tip: Firewall objects use as metadata variable

To ensure that only one remote site is connected at any given time in an IPsec VPN scenario, you should use route-overlapwith the option to either use-new or use-old. This setting dictates which routes are preferred and how overlaps in routes are handled, allowing for one connection to take precedence over the other (C).
References:
* FortiOS Handbook - IPsec VPN

To use the VPN interfaces in a policy, you need to install the VPN community and gateway configuration on the FortiGate devices first. This will create the VPN interfaces on the FortiGate and sync them with FortiManager. Reference:
Creating IPsec VPN communities
VPN | FortiGate / FortiOS 7.2.0

Option D is the correct answer because it specifically blocks access to the website "www.eicar.org" using TCP protocol and HTTP service, which are commonly used for web browsing. The other options either use the wrong protocol (UDP), the wrong service (DNS or SSL), or the wrong pattern ("eicar" instead of "www.eicar.org"). Reference := Configuring custom signatures | FortiGate / FortiOS 7.4.0 - Fortinet Document Library, section "Signature to block access to example.com".

The configuration line "set dpd on-idle" indicates that dead peer detection (DPD) is set to trigger only when the tunnel is idle, not actively disabled1. Reference: FortiGate IPSec VPN User Guide - Fortinet Document Library