Answer
5 N only.
Work Step by Step
If the two vectors point in the same direction, they can have a maximum of 7 N.
If they point in opposite directions, they can have a minimum of 1 N.
Pointing them at right angles gives a resultant of 5 N, by the Pythagorean theorem.
$$(3 N)^{2}+(4 N)^{2} =F^{2}$$
$$F=5 N$$
