(A) says refined-gold coins are inconsistent or unreliable in their value because the purity of refined gold varies. This explains merchants’ preference for Senegalese gold, which does not need to be refined and therefore lacks these inconsistencies, making it more reliable.

The prevalence of silver coins in trade transactions does not explain why merchants often specified that payment should be in the coins minted from Senegalese gold.

The mints’ ability to determine the value of gold coins does not explain why merchants, who are unrelated to the mints, would often specify that payment should be in the coins minted from Senegalese gold.

Payments commonly being made using coins minted in different countries does not explain merchants’ preference for coins minted from Senegalese gold. The question stem does not say the merchants preferred coins minted in Spain; they preferred coins minted from Senegalese gold.

This is not helpful for explaining the merchants’ preference. (E) seems to imply that merchants would have a preference against Senegalese gold coins, as Senegalese gold was never refined and could therefore not be resold for use in jewelry.