What does the MERGE syntax do?

Consider this syntax :
MERGE INTO t1
USING t2 ON (join predicate)?.
What doesthe MERGE syntax do?

A.
It performs a merge join of the row from T2 only if it doesn’t exist in the T1 table

B.
It creates a natural join of tables T1 and T2 for all columns that have the same name

C.
It creates a Cartesian product of table T1 and table T2 for all columns that have the same name

D.
For each row from T2, it updates the row if it exists within table T1, otherwise it inserts the row into T1