The correct answer is A. Minimum weight.
The economical depth of a plate girder is the depth that minimizes the weight of the girder while still meeting all of the design requirements. This depth can be found by using a formula that takes into account the loads on the girder, the material properties of the girder, and the desired safety factor.
The other options are incorrect because they do not take into account all of the factors that affect the weight of a plate girder. Option B, minimum depth, would not necessarily result in the minimum weight girder, as a shallower girder may require more material to resist the loads. Option C, maximum weight, would obviously result in the heaviest girder, which is not necessarily the most economical. Option D, minimum thickness of web, would also not necessarily result in the minimum weight girder, as a thinner web may not be able to resist the loads.
In conclusion, the economical depth of a plate girder corresponds to the minimum weight that meets all of the design requirements.