Oracle Database Discussions

Hello there guys, i´m studying to try a OCA certification, practing about 300 questions of SQL.
I have this question that i don´t understood very well. Please, someone help me with this issue:


"which constraint can be define only at the column level?"
a- unique
b- not null
c- check
d- primary key
e- foreign key

I already know the answer, but i would like to know why.

Someone could answer and explain it to me ?

thanks