Which method would serve the purpose?

You have an ORDERS table with the following structure:
Name Null? Type
————————- ——– ————————-
OID NUMBER(6)
ODATE DATE
CCODE NUMBER(6)
OAMT NUMBER(10,2)
The table has data in the ODATE column for all rows. Many orders are placed in a single day. You need to ensure that the ODATE column must contain data for every order in future.
Which method would serve the purpose?

A.
Add a PRIMARY KEY constraint to the column using the ALTER TABLE … ADD CONSTRAINT command

B.
Add a UNIQUE constraint to the column using the ALTER TABLE … ADD CONSTRAINT command

C.
Add a NOT NULL constraint to the column using the ALTER TABLE … ADD CONSTRAINT command

D.
Modify the column using the ALTER TABLE … MODIFY command