The wash-sale rule prevents an investor from doing which of the following after selling a stock at a loss?

The wash-sale rule is designed to prevent investors from claiming a tax deduction on a loss if they repurchase the same security within a specified time frame—specifically, within 30 days before or after the sale that generated the loss. If an investor sells a stock at a loss and then buys it back within this 30-day period, the IRS disallows that loss for tax purposes. The intent of the rule is to discourage "tax-loss harvesting," where investors could sell losing positions solely to create a tax advantage while still maintaining their investment in the same securities.

Thus, the correct answer highlights that an investor is prohibited from purchasing the same stock within a 30-day period after selling it at a loss to ensure that losses are not improperly claimed for tax deductions. This reinforces the importance of maintaining accurate tax reporting and compliance with IRS regulations.