The ORDER BY clause is used in a SELECT statement to sort the returned rows by one or more columns.
A). This clause would attempt to order by ename as if it were a numeric column, which is not correct since ename is not numeric.
B). This is the correct answer. The NVL function replaces nulls with the specified value, in this case, 0. This clause sorts by comm, with nulls considered as 0 and appearing first, followed by the actual values, and then sorts by ename.
C). This clause is incorrect because it would place null values at the end, but in the output, rows with null comm appear at the beginning.
D). This clause would sort the comm in descending order with the nulls at the end, which is not consistent with the output shown.
References:
* Oracle Database SQL Language Reference, 12c Release 1 (12.1): "ORDER BY Clause"
* Oracle Database SQL Language Reference, 12c Release 1 (12.1): "NVL Function" Please note that the correct formatting of SQL statements and clauses is crucial for the successful execution of queries.