To improve soil that has too much sand for growing plants, Carlos should add organic matter to enhance water retention and nutrient content. The best option from the given choices would be B. clay. Clay can help bind the sandy soil particles together, improving its structure and water retention capabilities.
However, it's also common to add organic matter such as compost or well-rotted manure, which are not listed in the provided options, but clay is the most suitable choice among them.