The correct answer to your question would be D - A trade policy that encourages exports while discouraging imports, mainly through tariffs.
The reason why this is the answer is because protectionism in its essence tries to restrain, limit and make trade less prominent between two nations by doing exactly that; encouraging exports and discouraging imports. This can be done in many ways. The idea behind this is that this serves and protects the interest of one nation (the protectionist one) and "evens out" the trading playing field.
"A. Coal was the main source of energy during the Industrial Revolution" is not true of coal production during the Industrial Revolution, since it was in fact the steam engine that superseded coal later in the Revolution.<span />
