Explanation
Correct answer: C.
Choice C is the best answer. “Consequently” logically signals that the postal problems described in this sentence (slow fee collection, heaps of undeliverable mail) were a consequence of the fee system described in the previous sentence.
Why the other choices are wrong
Choice A
Choice A is incorrect because “regardless” illogically signals that the postal problems described in this sentence occurred despite the fee system described in the previous sentence. Instead, they were a consequence of that system.
Choice B
Choice B is incorrect because “on the contrary” illogically signals that the postal problems described in this sentence contrast with the fee system described in the previous sentence. Instead, they were a consequence of that system.
Choice D
Choice D is incorrect because “for example” illogically signals that the postal problems described in this sentence are an example of the fee system described in the previous sentence. Instead, they were a consequence of that system.