A

To solve for the relationship between the vectors, we use the formula for the resultant of two vectors. Given the vectors \( \vec{A} \) and \( \vec{B} \) with an angle \( \theta = 120^\circ \), the magnitude of the resultant \( \vec{R} \) can be found using the formula:
\( R = \sqrt{A^2 + B^2 + 2AB \cos(\theta)} \)
With \( \theta = 120^\circ \), we have \( \cos(120^\circ) = -\frac{1}{2} \). Hence, the equation becomes:
\( R = \sqrt{A^2 + B^2 - AB} \)
We can infer that \( R \) must be equal to the maximum of the two magnitudes (due to the vector's triangle inequality) when \( A \) and \( B \) are equal or when the conditions are met. Therefore, the resultant must not exceed the individual magnitudes, confirming that option A is correct.