Multistage centrifugal pumps are used to obtain

Multistage centrifugal pumps are designed and used to obtain high head. These pumps are specifically engineered to provide high pressure or head, making them suitable for applications where the fluid needs to be pumped to a significant height or over long distances against high resistance or gravity.

Option A, "High discharge," is not the primary purpose of multistage centrifugal pumps. These pumps focus on generating high pressure (head) rather than delivering a high volume of fluid (discharge).
Option C, "Pumping of viscous fluids," is not a specific characteristic of multistage centrifugal pumps. The viscosity of the fluid being pumped can affect pump selection, but it's not the primary function of multistage pumps.
Option D, "High head and high discharge," is partially correct as multistage centrifugal pumps do provide high head, but they may not necessarily deliver a high discharge volume.

Therefore, the correct answer is Option B: High head. This is the primary purpose for which multistage centrifugal pumps are used.