The correct measures used to protect domestic firms against foreign competition and control the volume of imports include:
subsidies, import tariffs, and exchange controls
This choice includes various protective strategies such as providing financial assistance to domestic firms (subsidies), imposing taxes on imported goods (import tariffs), and regulating currency exchange to influence trade balances (exchange controls).
The other options, while they include some relevant concepts, are either inaccurate or less comprehensive in terms of traditional protective measures.