You must specify DROP FUNCTION to grant that privilege on a database object. If you are granting the privilege on a particular UDF only, then you can specify DROP without also specifying the FUNCTION keyword.
When creators have WITH GRANT OPTION on a function, they can grant the following privileges to another user on that function:
- DROP FUNCTION
- EXECUTE FUNCTION
Users with the DROP FUNCTION privilege on a function can also replace that function. If a user drops the function, however, the user cannot replace the function at a later time without also being granted the CREATE FUNCTION privilege.