The correct answer is: D. Column loads + full horizontal shear due to wind or earthquake.
The bracing between two columns of a steel tank is designed to resist the horizontal shear due to wind or earthquake. The horizontal shear is the force that acts perpendicular to the length of the tank. The column loads are the vertical forces that act on the columns. The bracing must be designed to resist the sum of the horizontal shear and the column loads.
Option A is incorrect because it only considers the horizontal shear due to wind or earthquake. Option B is incorrect because it only considers the column loads. Option C is incorrect because it considers the column loads and 2.5% of the horizontal shear due to wind or earthquake. This is not enough to resist the full force of the horizontal shear.
Option D is the correct answer because it considers the column loads and the full horizontal shear due to wind or earthquake. This is the most conservative option and will ensure that the bracing is able to resist the full force of the horizontal shear.