The resultant force of $5\text{N}$ and $10\text{N}$ cannot be

The correct option is C $4\text{N}$
When we vary the angle between two vectors, the magnitude of the resultant vector is minimum in the case when the two vectors are opposite to each other and maximum, when the two vectors are in same direction i.e. $(|\overrightarrow{F_1}-\overrightarrow{F_2}|<{\overrightarrow{F}}_{res}<|\overrightarrow{F_1}+\overrightarrow{F_2}|)$.
Thus, the magnitude of the resultant vector cannot be less than the minimum of the two vectors. Hence, the magnitude of the resultant vector cannot be less than $5\text{N}$.