The universal gravitational constant G was first measured experimentally by

The correct answer is B, Henry Cavendish. In 1798 Cavendish used a delicate torsion balance to measure the extremely small attraction between lead spheres in a laboratory and from it obtained a value of G, about 6.674 times ten to the power minus eleven newton metre squared per kilogram squared. Since G appears in the formula for g, the same experiment also gave the mass and mean density of the earth, which is why it is popularly called weighing the earth.

Option A, Isaac Newton, stated the law of universal gravitation in the seventeenth century but never measured G, which is why the constant remained unknown in his lifetime. Option C, Galileo Galilei, showed that bodies of different masses fall with the same acceleration and studied projectile motion, but did not work on G. Option D, Johannes Kepler, gave the three laws of planetary motion that Newton later explained using gravitation. Keep the pair Cavendish and G firmly apart from Newton and the law itself.