The correct answer is B. 2910 m.
The effective gradient on a runway is the ratio of the change in elevation to the horizontal distance over which the change occurs. It is expressed as a percentage. A 0.5% effective gradient means that the runway rises 0.5 meters for every 100 meters of horizontal distance.
When calculating the revised runway length, we need to take into account the effective gradient. The revised runway length is equal to the original runway length plus the product of the effective gradient and the original runway length.
In this case, the original runway length is 2845 m and the effective gradient is 0.5%. Therefore, the revised runway length is equal to 2845 m + (0.5% * 2845 m) = 2910 m.
Option A is incorrect because it is the original runway length. Option C is incorrect because it is the product of the effective gradient and the original runway length, but it does not take into account the original runway length. Option D is incorrect because it is the original runway length plus the product of the effective gradient and the original runway length, but it is too large.