The answer is standard.
A tariff is a tax imposed on imported goods and services. It is used to increase the cost of imports, making them less competitive compared to domestic products. In this case, the question does not mention any tax or financial charge being applied to the paint, so a tariff is not the correct answer.
A subsidy is a form of financial aid or support extended to an economic sector, generally with the aim of promoting economic and social policy. Subsidies are not related to prohibiting imports, so this option is not applicable to the scenario described.
A quota is a limit on the quantity of a good that can be imported into a country. While quotas restrict the amount of a product that can be imported, they do not outright prohibit the importation of a product based on its characteristics, such as containing lead.
A standard is a set of criteria or requirements that products must meet to be allowed for importation. In this case, the United States is prohibiting the importation of paint that contains lead, which is a safety standard to protect consumers from harmful substances. Therefore, the correct trade barrier being utilized is a standard.