There are several ways to write a SELECT statement that joins tables, but the standard and flexible approach is to specify the name of the table to be joined in the JOIN clause, and then write a conditional expression for the join in the ON clause. In the given example, the dept table is joined to the emp table, with emp.dept_id = dept.dept_id specified as the join condition. Therefore, the keyword that should fill the blank is "ON".

So the correct answer is C.

If two tables use the same column names, as in this question, you can use the USING keyword for convenience, followed by just the column names in list format. That is, you would specify it as JOIN dept USING (dept_id). Note that the column name must be enclosed in parentheses in this case.

If the only columns with the same name in the two tables are the ones you want to join, a simpler alternative is to specify NATURAL JOIN dept. In the example, the columns of employee name and department name are named differently (emp_name and dept_name, respectively), but it's common to see implementations where both are simply named "name". Be aware that if you use NATURAL JOIN in such a case, the "name" column will also be used for the join.