Hydrostatic Pressure:
Hydrostatic pressure is the pressure exerted by a fluid at any given point due to the weight of the fluid above it. It is the pressure exerted by a non-moving or static fluid column.

Explanation:
When a fluid is at rest, the pressure at any point within the fluid is the same in all directions. This is because the weight of the fluid above that point exerts a force on it, creating pressure. This pressure is known as hydrostatic pressure.

Key Points:
- Hydrostatic pressure is the pressure exerted by a fluid at any given point.
- It is caused by the weight of the fluid column above that point.
- The pressure at any point within a non-moving fluid is the same in all directions.
- It is a fundamental concept in fluid mechanics and is used to understand various phenomena, such as buoyancy and fluid statics.

Comparison with other options:
a) Gauge Pressure: Gauge pressure is the pressure measured relative to atmospheric pressure. It is the difference between the absolute pressure and the atmospheric pressure. It is not the pressure at a specific point in a non-moving fluid.
b) Atmospheric Pressure: Atmospheric pressure is the pressure exerted by the Earth's atmosphere on any object or surface. It is the pressure exerted by the air column above that point. It is not the pressure at a specific point in a non-moving fluid.
c) Differential Pressure: Differential pressure is the difference in pressure between two points in a fluid system. It is not the pressure at a specific point in a non-moving fluid.

Conclusion:
The correct answer is option 'D', hydrostatic pressure. It refers to the pressure exerted by a non-moving fluid at any given point due to the weight of the fluid column above that point. It is a fundamental concept in fluid mechanics and is used to understand various phenomena in the field.