The correct answer is: C. Decreased by 10% for bars in compression.
The average permissible stress in bond for plain bars in tension is 250 MPa. However, for bars in compression, this value is reduced to 225 MPa. This is because the bond between the concrete and the steel bar is weaker in compression than in tension. This is due to the fact that the concrete is more likely to crack under compression, which can break the bond between the concrete and the steel bar.
Option A is incorrect because the average permissible stress in bond for plain bars in tension is not increased by 10% for bars in compression. In fact, it is decreased by 10%.
Option B is incorrect because the average permissible stress in bond for plain bars in tension is not increased by 25% for bars in compression. In fact, it is decreased by 10%.
Option D is incorrect because the average permissible stress in bond for plain bars in tension is not decreased by 25% for bars in compression. In fact, it is decreased by 10%.