Two concurrent forces of 6 newtons and 12 newtons could produce the same effect as a single force of?

If the forces are acting in exactly opposite directions, they will subtract, and the magnitude of the resultant force is 6 N. This is the minimum resultant force, so that rules out (1). Similarly, if they act in exactly the same direction, they will add, and the resultant force is 18 N. This will be the maximum resultant force. That rules out (3) and (4). Thus, at some angle (between 0 and 180 degrees), you COULD find a resultant force of 15 N.