The correct answer is C. 210 cm.
The minimum headroom over a stair must be 210 cm, or 6.9 feet. This is to ensure that people can walk up and down the stairs without hitting their heads. The International Residential Code (IRC) requires that the headroom over a stair be at least 210 cm, measured from the top of the stair tread to the ceiling or any obstruction.
Option A, 200 cm, is too low. This would not allow people to walk up and down the stairs without hitting their heads.
Option B, 205 cm, is slightly higher than the minimum requirement, but it is still not enough to allow people to walk up and down the stairs without hitting their heads.
Option D, 230 cm, is too high. There is no need for the headroom to be this high, as it would not provide any additional benefit.
Therefore, the correct answer is C. 210 cm.