Friction provides centripetal force for the turn

Steps:

• Identify the need for centripetal force to change direction in a curve.
• Recognize that leaning balances gravitational torque with frictional torque.
• Note the horizontal component required for circular motion comes from tire-road interaction.
• Conclude friction supplies the inward force to turn left.

Why A is correct:

• Friction between tires and road provides the centripetal force $F_c = \frac{mv^2}{r}$, directing the motorcycle toward the curve's center per Newton's second law.

Why the others are wrong:

• B: Gravity acts vertically downward and cannot provide the horizontal centripetal force needed for turning.
• C: Air resistance opposes forward motion but does not cause the lateral turn.
• D: Normal force is perpendicular to the road (vertical) and supports weight without horizontal turning component.

Final answer: A