From Ron:
two problems with (d).
1. the meaning of the sentence is that the number of trucks in 1929 was almost twice 469,000.
choice (b) conveys this idea accurately, with the phrase "twice that number". see, "that number" must refer to an actual number cited in the sentence, and there is only one such number. mission accomplished.
choice (d) DOES NOT convey this idea. that choice says "twice AS MANY trucks", which means "twice as many trucks as were produced in 1914".
the problem is that the sentence doesn't tell us how many trucks were produced in 1914 - the only figure given is a combined figure for cars and trucks - so this statement doesn't make any sense in context.
not to mention, the intended meaning (from the original flawed sentence) is clearly that of (b).
2. "trucks themselves" doesn't make sense.
you don't use "x itself" unless you are trying to emphasize some element of the inherent nature of x (as opposed to something associated with x, or with some part of x).
example: pet accessories are becoming more and more popular, even though pets themselves have maintained constant popularity.
i.e., we want to emphasize that the second part of the sentence deals with pets themselves (as opposed to associated things such as pet accessories).
"trucks alone", though, makes perfect sense.