Explanation:

When a fluid flows through a screen, the direction of flow affects the behavior of the fluid and the screen itself. Screens are inclined to the direction of flow in order to achieve certain objectives.

Increase the Flow Velocity:
If the screens were inclined in the direction opposite to the flow, it would lead to a decrease in the flow velocity. This is because the fluid would have to climb against the incline of the screen, which would result in additional resistance. Therefore, option 'a' is incorrect.

Increase the Opening Area:
Inclining the screens in the direction of flow does not directly affect the opening area of the screen. The opening area of the screen is determined by the design and mesh size of the screen, and the inclination does not alter this. Therefore, option 'b' is incorrect.

Decrease the Flow Velocity:
When screens are inclined to the direction of flow, the flow velocity decreases. This occurs because the inclination of the screen causes the fluid to flow over a longer path compared to a flat screen. As a result, the fluid experiences more resistance and the flow velocity decreases. Therefore, option 'c' is correct.

Increase the Head Loss:
Head loss refers to the amount of energy lost due to fluid flow through a screen. Inclining the screens in the direction of flow does not directly increase the head loss. Head loss is primarily influenced by factors such as the design of the screen, the flow rate, and the properties of the fluid. Therefore, option 'd' is incorrect.

In conclusion, screens are inclined to the direction of flow in order to decrease the flow velocity. This inclination increases the resistance experienced by the fluid, resulting in a lower flow velocity.