Rotodynamic pumps are classified by the direction in which fluid leaves the impeller, and this direction fixes the head-versus-flow behaviour. The classification is captured by the specific speed, a shape parameter that is low for high-head machines and high for high-flow machines.

In a radial flow (centrifugal) pump, fluid enters axially at the eye of the impeller and is flung radially outward. The impeller does work mainly by centrifugal action, converting kinetic energy into pressure in the volute/diffuser. Turning the flow through a large radius change builds a high pressure rise but moves a comparatively small volume, so a radial machine has a low specific speed.

Therefore radial flow pumps are preferred when low flow, high head is required — for example boiler feed pumps, high-rise building water supply, and chemical injection, all of which need large pressure but modest discharge.

The comparison across impeller types makes this clearer:

This is why the option high flow, low head is wrong — that describes an axial (propeller) pump used in irrigation and flood control, the opposite of a radial machine. The option about very viscous flow is wrong because thick, viscous liquids are handled far better by positive-displacement pumps (gear, screw, lobe), whose delivery does not collapse with viscosity as a centrifugal pump's does. The option about low suction head available concerns cavitation and NPSH, which is a different design constraint and does not by itself make a radial pump the preferred choice.