Correct option is C
Explanation:
Conclusion I follows because experts directly recommend improving drainage.
Conclusion II is illogical (more dams will not solve the problem).
Conclusion III not follows this is a fact/effect stated in the passage, not a step or action to be taken. It should not qualify as a course of action.
Conclusion IV is partly true but too absolute, the statement says poor drainage is the main cause, not that rainfall plays no role.