The correct answer is: C. Uniform demand of resources is achieved.
Resource leveling is a technique used in project management to ensure that the demand for resources is as uniform as possible throughout the project. This can be done by delaying or accelerating tasks, or by adding or removing resources. Resource leveling can help to reduce costs, improve efficiency, and avoid delays.
Option A is incorrect because resource leveling does not necessarily reduce the total duration of the project. In fact, it can sometimes increase the duration if it requires tasks to be delayed or accelerated.
Option B is incorrect because resource leveling does not necessarily increase the total duration of the project. In fact, it can sometimes decrease the duration if it allows tasks to be completed sooner.
Option D is incorrect because resource leveling does not necessarily control the cost of the project. The cost of the project can be affected by a number of factors, including the cost of resources, the duration of the project, and the complexity of the project.
In conclusion, the correct answer is: C. Uniform demand of resources is achieved.