ANS:D - all the above.

1. "The force of friction always acts in a direction opposite to that in which a body is moving": This statement represents a fundamental principle of friction. Friction always acts in a direction opposite to the direction of motion or the direction in which a body tends to move. It opposes the relative motion between two surfaces in contact.
2. "The magnitude of the kinetic friction bears a constant ratio to the normal reaction between two surfaces. The ratio being slightly less than that in the case of limiting friction": This statement refers to the relationship between kinetic friction (friction between two surfaces in relative motion) and the normal force (the force exerted by a surface perpendicular to the contact surface). The magnitude of kinetic friction is approximately proportional to the normal force, with a coefficient of kinetic friction. This coefficient is generally slightly less than the coefficient of static friction, which represents the frictional force just before an object begins to move (limiting friction).
3. "For moderate speeds the force of friction remains constant but decreases slightly with the increase of speed": This statement describes the behavior of friction at moderate speeds. At moderate speeds, the force of friction tends to remain relatively constant but may decrease slightly as the speed increases. This decrease in frictional force at higher speeds can be attributed to factors such as the reduction of interlocking between surface asperities and the effects of lubrication or fluid film formation.

• The first statement represents a general principle of friction regarding its direction.
• The second statement describes the relationship between kinetic friction and the normal force, including the concept of limiting friction.
• The third statement characterizes the behavior of friction at moderate speeds, emphasizing its relative constancy with slight decreases as speed increases.