For more than 2,000 years, the only known sources of diamonds were categorized as what?

The correct choice highlights that for over 2,000 years, diamonds were primarily sourced from alluvial deposits. Alluvial diamonds are those that have been eroded from their original source rock, typically kimberlite, and are found in riverbeds, beaches, or along ocean shorelines. This natural process allows diamonds to be located in sedimentary layers where they have been transported by water.

Historically, these alluvial deposits were the most accessible places for early diamond hunters to find gems, leading to the substantial trade and cultural significance of diamonds long before modern mining techniques were developed. This method of sourcing diamonds laid the foundation for the diamond industry's growth and the establishment of diamond trading routes.

In contrast, marine, synthetic, and inefficient do not accurately define the primary sources of diamonds during that extensive historical period. Marine pertains to oceanic sources, synthetic refers to man-made diamonds which began making significant appearances much later, and inefficient doesn't specifically relate to a category of diamond sources.