which server should you run adprep.exe?

70-412 As DC3 is the only server running 2008 or above and the 64 bit version of adprep for server 2012 or above must be run on a server that meets those requirements the answer is correct although a little vague as it doesn’t specify if you are running /forestprep (which must be run on the Schema master) or /domainprep (which must be run on the infrastructure operations master for the domain)
Hopefully the question will be clearer if there is a similar one in the exam

Beginning with Windows Server 2012, Adprep.exe is integrated into the AD DS installation process and runs automatically as needed. For example, when you install the first domain controller that runs Windows Server 2012 into an existing domain and forest, then adprep /forestprep and adprep /domainprep automatically run and report the results of the operations.

Some organizations may prefer to run Adprep.exe separately, either in advance of an AD DS installation or simply to extend an existing AD DS schema to support new features such as the Device Registration Service in Windows Server 2012 R2. For this reason, Adprep.exe is also included in the \Support\Adprep folder of the operating system disk.

Also beginning with Windows Server 2012, there is only one 64-bit version of Adprep.exe. It can be run remotely from any server that runs a 64-bit version of Windows Server 2008 or later. The computer where you run it can be either domain-joined or in a workgroup. It includes new syntax and parameter options in order to run it remotely.

In Windows Server 2008 R2, Adprep.exe is located in the \Support\Adprep folder of the operating system disk. In Windows Server 2008, Adprep.exe is located in the \Sources\Adprep folder.

Windows Server 2008 R2 includes a 32-bit version and a 64-bit version of Adprep.exe. The 64-bit version runs by default. If you want to run one of the Adprep.exe commands on a 32-bit computer, use the 32-bit version of Adprep.exe (Adprep32.exe).