Answer:
A. Adhering to taxation rules.
Step-by-step explanation:
This is because, when there is no level playing field for different segments of the business in terms of U.S tax policy, there will surely be challenges in "Adhering to taxation rules."
Given that the statement might imply that there are different rules applicable to different sectors of business, this will make it difficult for the different firms or players involved to follow the taxation rules.
Hence, stating that many believe that U.S. tax policy should provide a level playing field for traditional retail businesses, mail order companies, and online merchants, indicated that the e-business challenge is "Adhering to taxation rules."