The correct answer is option C) Energy grade line.

Explanation:
The energy grade line (EGL) is the locus of elevations that water will rise in a series of pitot tubes. It represents the total energy of the fluid at a particular point in a fluid flow system.

To understand the concept of the energy grade line, let's break down the components of the total energy of a fluid:
1. Elevation head (Z): This is the potential energy of the fluid due to its height above a reference point, typically a horizontal plane. It is given by the equation Z = h * g, where h is the height above the reference point and g is the acceleration due to gravity.

2. Pressure head (P): This is the energy associated with the pressure of the fluid. It is given by the equation P = p / (ρ * g), where p is the pressure of the fluid, ρ is its density, and g is the acceleration due to gravity.

3. Velocity head (V^2 / (2g)): This is the kinetic energy of the fluid due to its velocity. It is given by the equation V^2 / (2g), where V is the velocity of the fluid and g is the acceleration due to gravity.

The total energy (H) of the fluid at a particular point can be expressed as the sum of these three components:
H = Z + P + V^2 / (2g)

The energy grade line represents the variation of total energy along a fluid flow system. It is a graphical representation of the total energy at each point along the flow path. The EGL is typically shown as a line on a plot of elevation (Z) versus distance along the flow path.

The EGL is important in fluid mechanics as it helps engineers analyze and design fluid flow systems. By comparing the EGL at different points, engineers can determine the pressure variations, identify areas of head loss, and optimize the system for efficient flow.

In summary, the locus of elevations that water will rise in a series of pitot tubes is called the energy grade line (EGL). It represents the total energy of the fluid at different points along the flow path.