The idea here is that "were" and "did" can stand in for a previous verb, just like pronouns can stand in for a previous noun. "Were" typically stands in for another use of "was" or "were," or, if there has been a shift in time, for another form of "to be":
My parents were not as excited about the trip as my sister and I were. ("Were" stands for "were excited.")
The current generation of entrepreneurs will not be as successful as their predecessors were. ("Were" stands for "were successful," referring back to the earlier "will . . . be . . . successful.")
Do/does/did is more all-purpose. It can stand in for any other verb that represents something you would do:
I like pie more than my wife does. (does=likes pie)
He scoffed at the idea, as did most of the other investors. (did=scoffed at the idea)
With all this in mind, let's look at B and D:
B) Today, because of improvements in agricultural technology, the same amount of acreage produces
twice as many apples as it did in 1910.
Did = produce. A certain acreage produces twice as many apples as that same acreage did (produce) in 1910. This works fine.
D) Today, because of improvements in agricultural technology, the same amount of acreage produces
two times as many apples as there were in 1910.
Here, "were" stands in for what? There is no previous use of "were," or any other form of "to be," for that matter. That might be okay, as we are actually using a common comparison expression. The real meaning is that there are twice as many apples now as there were in 1910. The problem is that we aren't comparing the same things. We are comparing what a certain acreage
produces now to how many apples
existed in 1910. This doesn't make sense. It would be like saying "The average bond trader today earns twice as much as there was in 1940." Unless we mean that the average bond trader makes more money than there was in the whole world in 1940, this doesn't work. And if that is what we mean, I think I'll retire from gmatclub and take up bond trading.