Two vectors $\vec{a}$ and $\vec{b}$ are drawn from the same starting point as adjacent sides of a parallelogram. According to the parallelogram law of vector addition, the resultant $\vec{a} + \vec{b}$ is:

Parallelogram law: when two vectors emerging from the same point are taken as adjacent sides of a parallelogram, the diagonal drawn from that point represents the resultant in both magnitude and direction. Option D is only true when $\vec{a}$ and $\vec{b}$ are parallel and pointing the same way. In general, $|\vec{a} + \vec{b}| \leq |\vec{a}| + |\vec{b}|$, with equality only in the parallel case.