In which condition the acceleration due to garvity is zero

When the object is infinitely far away from the planet, or when the planet's mass is zero (which is much less likely).

Explanation:

Using the formula for gravitational force:

Fg = GMm/r^2

Using Newton's 2nd law, F=ma, we can rewrite this equation as

mg = GMm/r^2

cancel m from both sides

g = GM/r^2

the acceleration due to gravity is zero when the mass of the planet is zero, or the object is an infinite distance away from the planet. G is a constant.

it takes about 30 days for it to break down

d.7.5cm is your answer