@mortonsalt, please clarify if I've misunderstood what you said above, bus far as I can tell, it's ok to add the two force vectors to find the a third vector that will produce equilibrium. This is ok as long as you realize that you need to reverse the DIRECTION of the resultant vector in order to produce the equilibrium vector you want. This isn't a big deal for this question because you're only asked to find magnitude.
For instance, If you wanted to balance each force separately, you would just add an equal but opposite force. Add those two opposite forces together, and you get the equilibrium vector that is equal in magnitude but opposite in direction to the resultant vector of your original two forces.
Also, since your starting forces have whole number magnitudes plotted on a convenient grid, you don't need to consider angles at all. If you add the two vectors in the diagram, the resultant can be deconstructed into a right triangle that has a (+2)+(+4)=+6N x-component and a (+3)+(-1)=+2N y-component. By the Pythagorean Theorem, the hypotenuse (i.e. the magnitude of the resultant) is about 6.3N (answer C). The equilibrium vector would also have this magnitude, but would have exactly the opposite direction (i.e. -6N x-component and -2N y-component).