Questions

For an atom to emit X-rays it should have difference in energy levels

Water has the maximum density at 4oC. So at this temperature, it sinks down to bottom and stays here. If it cools further, it will become less dense and move up. So while the upper layers can cool enough to form ice, the bottom layers remain as water at  4oC .  

Therefore, (C) is correct.

The Rolling plan refers to one-year plans in the five-year perspective. under Rolling plan, there would be an annual appraisal of the progress of the plan and on the basis of such an appraisal, the plan for the next year will be prepared. The plan would go on rolling year after year.