ANS:D - all 'a', 'b' & 'c'.

Cascade control is useful to control flow from temperature. Cascade control involves using the output of one controller to manipulate the setpoint of another controller. It is commonly used in situations where a secondary variable is affected by changes in a primary variable, and controlling the secondary variable directly is difficult or impractical. In the context of the options provided:

• Option a describes the concept of setpoint adjustment by a primary (master) controller, which is not specifically related to cascade control.
• Option b refers to the use of cascade control when changes in process conditions cause serious upsets in the controlled variable. While cascade control can help mitigate such upsets, this option does not fully describe the concept of cascade control.
• Option c describes a scenario where cascade control may be implemented, but it does not fully explain the concept of cascade control itself.

Cascade control is most closely associated with option c, as it involves controlling one variable (such as flow) based on the measurement of another variable (such as temperature). Therefore, the correct answer is all 'a', 'b' & 'c'. However, it's important to note that option c does not fully describe cascade control.