The correct answer is option 4: A is false, but R is true.

Option 1 suggests that both the assertion (A) and the reason (R) are true, and that the reason correctly explains the assertion. However, this is not correct.

Option 2 states that both the assertion and the reason are true, but the reason is not the correct explanation of the assertion. This is also incorrect, as the reason is indeed a correct explanation of the assertion.

Option 3 suggests that the assertion is true, but the reason is false. However, this is also incorrect, as the reason is true.

Option 4 is the correct answer, as the assertion is false. When a piece of rock is brought from the moon to the earth, its mass does not change. However, the reason is true - the gravitational pull of the earth is indeed stronger than that of the moon.

In summary, while the reason (R) is true, the assertion (A) is false. The gravitational pull of the earth does not change the mass of a rock brought from the moon to the earth.