Which partition to make active?
Solution 1:
The partition flagged "active" should be the boot(loader) one. That is, the partition with BOOTMGR (and the BCD) on it.
On a typical fresh Windows 10 installation, this would be the "System Reserved" partition, yes.
Of course, this only applies to MBR disks (booted in BIOS/CSM compatibility mode). GPT disks should instead be using a EFI System Partition, identified by the partition ID rather than any "active" flag. Windows can only boot GPT disks in UEFI mode.