Final answer:
The correct answer is D. Dishonesty, as historical records show that tax collectors often engaged in corrupt practices, undermining the effectiveness of export taxes.
Step-by-step explanation:
The taxes placed on exports were ineffective because of smuggling and because of the inefficiency and sometimes dishonesty of tax collectors. This choice is supported by historical instances where tax enforcement was compromised due to corruption and smuggling activities. For example, during the time of the Southern Dynasties, officials earned reputations for corruption, which included manipulating trade to their advantage. Similarly, during the colonial period, smuggling was a common reaction to the British imposition of trade taxes, leading to the ineffectiveness of such taxes. Furthermore, the reluctance of governments to appear morally compromised sometimes led them to forego direct taxation on trade, as was the case with the Tang Dynasty which sought to maintain an image of a moral government by avoiding the direct taxation of maritime trade.