Final answer:
The correct option is A. To find the frictional force without the coefficient, use Newton's laws. Construct a free-body diagram and apply Newton's second law (F = ma) to solve for the unknown frictional force.
Step-by-step explanation:
To determine the frictional force without knowledge of the coefficient of friction, one must rely on Newton's laws of motion. Specifically, Newton's first law tells us that a body at rest or in uniform motion will remain so unless acted upon by a net external force. Friction is an external force acting on a system. To solve for the frictional force, a free-body diagram should be drawn to identify all external forces acting on your system.
Once the free-body diagram is constructed, Newton's second law is invoked to solve for the frictional force; if all forces and accelerations are known except for friction, the frictional force can be deduced from the equality of the net force and the product of mass and acceleration (F = ma).