In a before insert trigger:
Trigger.old is null.
There is no existing record in the database to reference.
Why Not Other Options:
Option A: Describes Trigger.new.
Option B: The variable is defined but has a null value.
Option D: Trigger.old is not an empty list; it is null.
Reference:
Trigger Context Variables:
"In insert triggers, Trigger.old is null because the records are new."
- Apex Developer Guide: Trigger Context Variables