Final answer:
To increase the flow rate, it is generally best to increase the pressure. Increasing viscosity or decreasing the pipe diameter can actually impede flow, and decreasing temperature may or may not increase flow depending on other factors.
Step-by-step explanation:
To increase the flow rate for a process, the most effective change would usually be to increase the pressure driving the fluid through the system. Decreasing the temperature could cause some fluids to become more viscous, which would actually inhibit flow. Increasing the viscosity directly would also slow down the flow, since viscosity is a measure of a fluid's resistance to flow. Decreasing the pipe diameter would increase the fluid velocity due to conservation of mass (continuity equation), but it does not necessarily increase the flow rate, which is a volume per time measurement, and could also create greater resistance to flow.
From the information given, options (c) 'Increase the viscosity' and (d) 'Decrease the pipe diameter' are changes that would usually decrease the flow rate, not increase it. Option (b) 'Decrease the temperature' might either increase or decrease the flow rate, depending on the specific fluid and the temperatures involved, but in general is less likely to result in increased flow compared to increasing pressure. Therefore, the best answer is (a) 'Increase the pressure', which directly contributes to an increase in flow rate as per Bernoulli's equation which states that an increase in the fluid's potential energy per unit volume results in increased flow rate.