Which tax system applies a flat rate on retail transactions?

The correct choice is indeed Sales Tax because it is specifically designed as a tax levied on retail transactions. This tax is applied to the sale of goods and services, typically calculated as a percentage of the purchase price. Sales tax is generally uniform, meaning that the same rate is applied regardless of the buyer's income or wealth, which is why it is considered a flat rate tax.

In contrast, income tax operates on a progressive system where the rate increases with higher income levels. Property tax is based on the assessed value of an individual’s real estate and not a rate applied to transactions. Meanwhile, capital gains tax is imposed on the profit made from selling assets or investments, also differing from the transactional focus of sales tax. Hence, Sales Tax is the correct answer as it clearly fits the definition of applying a flat rate to retail transactions.