Step 1: State Newton's second law for a system of particles. The net external force on the system equals the system's total mass multiplied by the acceleration of its center of mass.
\[ \vec{F}_{net, ext} = M \vec{a}_{CM} \]
Step 2: Incorporate the given condition. The problem states that the net external force is zero.
\[ 0 = M \vec{a}_{CM} \]
Step 3: Determine the acceleration of the center of mass. Since the total mass \(M\) is non-zero, the acceleration of the center of mass must be zero.
\[ \vec{a}_{CM} = 0 \]
Step 4: Conclude from the result. Zero acceleration indicates that the velocity of the center of mass remains constant. This means the center of mass travels at a uniform velocity. Being "at rest" is a specific instance of constant velocity where that velocity is zero. Therefore, option (2) provides the more general and accurate description.