Explanation:

- Option 1: "Cash transactions had been replaced by credit cards."

- This uses past perfect tense. It implies a completed action before another past activity, which doesn’t fit the ongoing nature.

- Option 2: "Cash transactions have been replaced by credit cards."

- This uses present perfect tense. It indicates a completed action with relevance now but doesn't emphasize the process.

- Option 3: "Cash transactions are being replaced by credit cards."

- This uses present continuous tense. It suggests an ongoing process, fitting the scenario of a gradual change.

- Correct Answer

- Option 4: "Cash transactions are replaced by credit cards."

- This is simple present. It reads as a general fact, not focusing on the ongoing transition.