Which of Kepler’s laws implies that the gravitational force acting on a planet is a central force?
Kepler’s second law (law of areas) states that the line joining a planet to the Sun sweeps out equal areas in equal times. This is a consequence of the conservation of angular momentum, which holds true for a central force directed along the line joining the two bodies (e.g., the Sun and planet). A central force ensures no torque, preserving angular momentum.
Ref: Halliday & Resnick, Fundamentals of Physics, 11th ed., Chapter 13: Gravitation.