Which action can best prevent fraud at the checkout stage?

Adding a CAPTCHA system at the checkout stage is an effective way to prevent fraud because it helps distinguish between human users and automated bots. Bots can be programmed to abuse the checkout process, making fraudulent transactions at a high frequency without the limitations that a human user would face. By implementing a CAPTCHA, businesses can significantly reduce the risk of automated attacks, ensuring that only legitimate users can complete transactions. This layer of security is particularly important in preventing false transactions generated by scripts or malicious software that aim to exploit vulnerabilities in the checkout process.

While the other options might improve user experience or customer acquisition, they do not specifically target the prevention of fraud at the checkout stage. For instance, enforcing a minimum purchase requirement could deter some users but does not inherently address authentication or verification issues. Offering discounts for first-time buyers might attract more customers but could also encourage fraudulent sign-ups without proper checks in place. Providing alternative payment methods broadens payment options for users, but it doesn't directly prevent fraudulent behavior. Therefore, a CAPTCHA system stands out as a focused security measure that directly mitigates the risk of fraudulent transactions at checkout.