The correct answer is: C. 4.0 cm larger than the diameter of the column.
The diameter of the column head support a flat slab, is generally kept 4.0 cm larger than the diameter of the column. This is to ensure that the column head has enough area to distribute the load from the slab evenly. If the column head is too small, the load will be concentrated in a small area, which can cause the slab to crack or fail.
Option A is incorrect because the span length is not relevant to the diameter of the column head. The span length is the distance between the supports for the slab, and it does not affect the amount of load that the column head needs to support.
Option B is incorrect because the diameter of the column head is not proportional to the diameter of the column. The column head needs to be larger than the column in order to distribute the load evenly, but it does not need to be 0.25 times the diameter of the column.
Option D is incorrect because the column head does not need to be 5.0 cm larger than the diameter of the column. 4.0 cm is the minimum recommended size, and larger sizes may be necessary for certain applications.