In Oracle SQL, the ALTER TABLE statement is used to modify the definition of a table after it has been created. Specific changes are allowed depending on the current data type of the column and the data it contains.
A . You cannot change a VARCHAR2 column to NUMBER if the table contains rows with non-numeric values, which is the case with '123' as a VARCHAR2; it is a string, not a number.
B . You can reduce the size of a NUMBER column if the data will still fit within the new precision, but you cannot change a VARCHAR2 column to a smaller size if it contains values that exceed the new size limit.
C . You cannot change a NUMBER column to VARCHAR2 if the table contains rows because the existing data type of NUMBER does not match the new data type of VARCHAR2.
D . Similar to A, you cannot change the data type from VARCHAR2 to NUMBER if non-numeric data is present.
E . You can change a VARCHAR2(10) column to VARCHAR2(5) if all existing rows have values that fit within the new size, and you can change a NUMBER column to have a different scale (NUMBER(12,2)).
Reference:
Oracle Database SQL Language Reference, 12c Release 1 (12.1): "ALTER TABLE"