If a nation that does not allow international trade in steel has a domestic price of steel lower than the world price, then:

A. the nation has a comparative advantage in producing steel and would become a steel exporter if it opened up trade.
B. the nation has a comparative advantage in producing steel and would become a steel importer if it opened up trade.

A. the nation has a comparative advantage in producing steel and would become a steel exporter if it opened up trade. 

Explanation:

A country has comparative advantage in production if it produces at a lower opportunity cost when compared with other countries.

If the price of the country is lower than the world price, it indicates that the country has a lower opportunity cost and a comparative advantage in the production of the good.

A country that has comparative advantage in the production of a good, should specialise in its production and export it to other countries.

A closed economy is an economy that doesn't trade with other countries.

An open economy is an economy that trades with other countries.

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