Which type of agitation involves the use of propellers inside the tank?

Mechanical agitation is the correct answer because it specifically refers to the use of physical devices, such as propellers or paddles, to create movement and mix the contents within a tank. In this method, the mechanical action of the propellers effectively circulates the liquid and ensures a homogeneous mixture by physically stirring the product.

Hydraulic agitation typically relies on the flow of liquid under pressure to mix substances, which can differ significantly from the direct mechanical mixing provided by propellers. Centrifugal agitation involves the use of centrifugal force generated by moving components to achieve mixing, while Venturi agitation uses the principle of fluid dynamics in a tapered tube to create a vacuum that draws in and mixes additional materials, rather than relying on a direct mechanical action. Thus, the defining characteristic of mechanical agitation is its use of mechanical devices like propellers to achieve mixing, making it the correct answer.