The correct statement is:
a. the object is at rest
However, it's important to note that when the net force is zero, the object can also be in motion at a constant velocity (not necessarily at rest). Therefore, the more precise answer is that the object either remains at rest or moves at a constant velocity. If the object is at rest and remains at rest, or if it's in motion, it will continue to move in a straight line at constant speed when the net force is zero.