Fundamental unit operations in fluid processing:

• Mixing is the random intermingling of two or more separate phases into a more uniform composite state.
• Kneading is a high-viscosity deformation operation used for solid doughs and plastic pastes.
• Agitation strictly refers to the induced motion of a fluid material in a specified circulatory flow pattern inside a container or tank by means of an impeller, propeller, or paddle.
• Shearing is the application of velocity gradients across fluid layers.

Construction of centrifugal pumps:

• Pistons belong to reciprocating positive displacement pumps.
• The two vital working components of any centrifugal sanitary milk pump are the rotating Impeller (which imparts kinetic energy to the liquid) and the stationary Volute casing (a spiral expanding chamber that smoothly converts velocity head into pressure head).
• Diaphragms belong to diaphragm pumps.
• Gears belong to rotary gear pumps.

Diffuser-type centrifugal pumps:

• Piston pumps use cylinder valves and pistons.
• In diffuser-type centrifugal pumps (turbine pumps), stationary expanding guide vanes (the diffuser) surround the impeller periphery to gradually decelerate liquid exiting the impeller tips, converting kinetic energy into pressure with higher hydraulic efficiency than a plain volute casing.
• Screw pumps use intermeshing helical screws.
• Gear pumps use rotating meshed gears.

Operational characteristics of positive displacement (PD) pumps:

• Statement (A) is true: PD pumps operate efficiently against high discharge pressures with moderate/low flow rates.
• Statement (B) is true: flow rate in PD pumps is directly proportional to rotational/stroking speed ($Q \propto N$).
• Statement (C) is false: because displacement per stroke/revolution is fixed and independent of backpressure, PD pumps are universally used as precision metering and dosing pumps.
• Statement (D) is false: because a PD pump will continue delivering flow regardless of downstream line blockages, an overpressure relief valve is strictly mandatory to prevent pump and pipe burst.
  Since (C) and (D) are not true, Option 4 is the correct choice.