The correct answer is: Irrigation facilities.
Winter cropping in Manipur is possible in areas with irrigation facilities. This is because the winter months in Manipur are dry, and without irrigation, crops would not be able to grow. Irrigation facilities provide water to crops during the dry season, which allows them to grow and produce a yield.
The other options are not correct because they do not provide the necessary conditions for winter cropping in Manipur. High altitude does not necessarily mean that there is enough water for crops to grow, and mild temperatures can also be found in areas without irrigation facilities. Sandy soils are not ideal for growing crops, as they do not retain water well.
In conclusion, the correct answer is: Irrigation facilities.