If the angle between two forces increases, the magnitude of their resultant

1. first decreases and then increases
2. increases
3. remains unchanged
4. decreases

The correct option is D decreases
From the parallelogram law, the resultant of the force is given by
$F_{res}=\sqrt{F_1^2+F_2^2+2F_1{F}_2\cos \theta }$
As $\theta$ increase, $\cos \theta$ decreases. So, magnitude of resultant force decreases.