Why does the startup command return the error shown?

You issue commands in SQL*Plus as the Oracle owner, to enable multithreading for your UNIX- based Oracle 12c database:

CONNECT/AS SYSDBA
ALTER SYSTEMSETTHREADED EXECUTION= TRUESCOPE=SPFILE; SHUTDOWNIMMEDIATE

You then restart the instance and get an error:

STARTUP
ORA-01031:insufficientprivileges

Why does the startup command return the error shown?

A.
because the threaded architecture requires exiting from sql*pIus and reconnecting with sql*PIus /
as sysdba before issuing a startup command

B.
because the threaded architecture requires issuing a new connect / as sysdba from within sql*pIus before issuing a startup command

C.
because the threaded architecture requires authentication using a password file before issuing a startup command

D.
because the threaded architecture requires connecting to the instance via a listener before issuing a startup command

E.
because the threaded architecture requires restarting the listener before issuing a startup command