I don't think that pronoun ambiguity is the criterion here.

the sentence is supposed to say that today's small cars are more fuel-efficient than previous small cars -- i.e., OTHER small cars.
this is important, because you can't use a "they"/"their" construction (which would illogically imply that you're talking about the same small cars mentioned in the first part of the sentence).
the use of "those" in (c), on the other hand, accomplishes this distinction nicely.

analogy:

sprinters in texas can run faster than they can in north dakota.
--> illogical; in this sentence, "they" would be taken to mean "sprinters in texas" (i.e., not just sprinters).

sprinters in texas can run faster than those in north dakota.
--> makes sense; "those" = sprinters, in this case.

same thing with "they/their" vs. "those" in these instances.