The correct answer is: A. 12 bar diameters.
The length of lap in a compression member is kept greater than bar diameter x (Permissible stress in bar / Five times the bond stress). This is to ensure that the bar is able to transfer the load from one section of the member to the other without failing. The permissible stress in bar is the maximum stress that the bar can withstand without failing, and the bond stress is the stress that is developed between the bar and the concrete. The five times bond stress is a safety factor that is used to account for possible variations in the strength of the concrete and the bar.
If the length of lap is less than the required value, the bar may not be able to transfer the load from one section of the member to the other, and the member may fail. This is why it is important to ensure that the length of lap is always greater than the required value.
The other options are incorrect because they are not the required value for the length of lap in a compression member.