In terms of mercantilism, what is protectionism?
a. a political policy that states that a nation should take over foreign ports by force if necessary to insure healthy trading relations
b. an economic policy in which a nation spends a great deal of money on its navy in order to defend it merchant ships
c. a trade policy that involves undercutting the prices of other an nation's goods at all costs
d. a trade policy the encourages exports while discouraging imports, mainly through tariffs

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.