Why was rice chosen to serve as money in medieval China?

Answer 1

The way that a country's economy for trade typically develops is first by using a barter system, then by coins and precious metals, and then by fiat money.

Bartering has several problems, most notably the coincidence of needs problem and a wheat-field farmer need what the other produces a direct barter swap is impossible for seasonal fruit that would spoil before the grain harvest.

A solution is to indirectly trade fruit for wheat through a third, "intermediate", commodity: the fruit is exchanged for this when it ripens. If this intermediate commodity doesn't perish and is reliably in demand throughout the year (e.g. copper, gold, or wine) then it can be exchanged for wheat after the harvest.

The function of the intermediate commodity as a store-of-value can be standardized into a widespread commodity money reducing the coincidence of wants problem. By overcoming the limitations of simple barter, a commodity money makes the market in all other commodities more liquid. This is why the Chinese used rice and a standard measure of trade. The resource was readily available; widely (if not entirely) recognized and accepted.