During the High Middle Ages, an agricultural surplus played a significant role in expanding trade. In particular, the correct explanation is:
Trade increased because surplus crops could be traded.
When agricultural practices improved, and yields increased, regions with surplus crops were able to produce more than they needed for local consumption. This excess could then be traded with other areas, leading to the development of more extensive trade networks. Surplus agricultural products allowed for the exchange of goods, facilitating economic growth and interactions among different regions. Thus, the surplus supported not only local economies but also encouraged the growth of trade as communities exchanged goods and services at markets and fairs.