Which of these forms of payment is legally accepted?

Cash is legally accepted as a form of payment because it is recognized by governments as legal tender. This means that it is accepted for settling debts and obligations within an economy, backed by the government's authority. Cash transactions provide immediate payment and are often straightforward in terms of legal and regulatory considerations.

While barter, digital currencies, and trade credit may be viable methods of exchange in certain contexts, they do not have the same universal legal standing as cash. For example, barter relies on the mutual agreement of value between parties and lacks standardization. Digital currencies, while gaining acceptance, are still subject to varying regulations depending on jurisdiction, and their status as legal tender is not universally established. Trade credit, on the other hand, involves a promise to pay in the future and is not an immediate form of currency. Thus, among the options provided, cash stands out as the most clear-cut and universally accepted form of payment.