Is it possible to create an active full backup manually?

A.
Yes, but only via Powershell script.

B.
Yes, using the Active Full option from the shortcut menu of a corresponding job.

C.
No, active full backups can be created only according to the schedule.

D.
Yes, but only if the forward incremental backup mode is used.

Explanation:
https://helpcenter.veeam.com/backup/hyperv/active_full_backup.html