Decrease in distance between plates, all other factors constant.

Explanation:

Capacitance is a measure of a capacitor's ability to store electric charge. It is directly proportional to the physical characteristics of the capacitor, such as the area of the plates, the distance between the plates, and the dielectric constant of the material between the plates.

When all other factors are constant, the capacitance of a capacitor increases with a decrease in the distance between the plates. This can be explained by the following reasons:

1. Formula for Capacitance:
The capacitance of a parallel plate capacitor is given by the formula C = ε₀A/d, where C is the capacitance, ε₀ is the permittivity of free space, A is the area of the plates, and d is the distance between the plates. From this formula, it is clear that capacitance is inversely proportional to the distance between the plates (C ∝ 1/d).

2. Electric Field:
The electric field between the plates of a capacitor is directly proportional to the voltage applied across the plates and inversely proportional to the distance between the plates. When the distance between the plates is decreased, the electric field becomes stronger. This stronger electric field allows for a higher charge to be stored on the plates, resulting in an increased capacitance.

3. Overlapping Electric Fields:
When the distance between the plates is decreased, the electric fields produced by each plate overlap more extensively. This overlapping of electric fields leads to a higher capacitance as more electric flux is confined between the plates.

4. Dielectric Constant:
The dielectric constant of the material between the plates also affects the capacitance. However, in this question, it is mentioned that all other factors are constant. Therefore, the dielectric constant remains the same, and the change in capacitance can be attributed solely to the change in plate distance.

In conclusion, when all other factors are constant, the capacitance of a capacitor increases with a decrease in the distance between the plates. This is due to the inverse relationship between capacitance and plate distance, the stronger electric field between the plates, the overlapping electric fields, and the unchanged dielectric constant.