Explanation

An arbitrage opportunity must satisfy three key requirements:

1. Risk-free (A): Arbitrage involves no risk - the profit is guaranteed regardless of market movements

2. Zero net investment (B): True arbitrage requires no initial capital outlay - positions are self-financing through simultaneous buying and selling

3. Profitable (C): By definition, arbitrage must generate positive returns

However, return in excess of the risk-free rate (D) is NOT a requirement for arbitrage. While arbitrage opportunities often provide returns above the risk-free rate, this is not a defining characteristic. The essential feature is that the return is risk-free, not necessarily that it exceeds the risk-free rate. In efficient markets, arbitrage opportunities should be quickly eliminated, so they typically don't persist long enough to consistently provide returns above the risk-free rate.