The kings of Ghana became wealthy primarily through their control of trade, particularly in gold. They managed the gold trade by controlling the supply of gold available in the region, which allowed them to set prices and maximize profits. Additionally, they also engaged in the trade of other valuable commodities, such as salt, which was highly sought after in neighboring regions. The trans-Saharan trade routes played a vital role in this economic prosperity, as they facilitated the exchange of goods like gold and salt, as well as ivory and other products.
Among the options you provided, the most relevant would be "They controlled how much gold was available for trade," as this aspect of trade control was crucial to their wealth.