Which of the following statements is incorrect about gravitational force?
Gravitational force is always attractive (option 2 is correct), follows the inverse-square law (option 3 is correct), and acts between all masses (option 4 is correct). Option 1 is incorrect as it can be zero inside a shell.
Ref: Halliday & Resnick, Fundamentals of Physics, 11th ed., Chapter 13: Gravitation.