To understand which of the options is responsible for positive geotropism movement in plants, we must first define geotropism.

Geotropism: Geotropism, also known as gravitropism, is the growth movement of a plant or a part of a plant in response to gravity. In geotropism, stems typically show negative geotropism by growing upwards, against gravity, while roots demonstrate positive geotropism by growing downwards, in the direction of gravitational pull.

Now, let's evaluate each option in relation to geotropism:

1. Chemical: While chemicals (such as hormones like auxins) play a role in how plants grow, they are not the primary cause of the directional growth that defines positive geotropism.
2. Gravity: This is the force that plants respond to during geotropism. In positive geotropism, roots move toward the gravitational pull, demonstrating this effect.
3. Light: Light affects plant growth through phototropism, a different kind of stimulus-response where plants grow towards light.
4. Water: Water influences directional growth through a process known as hydrotropism.

Therefore, the correct answer based on the given options is Gravity, as it is the force responsible for the positive geotropism movement in plants.