Answer:
A line of reasoning that presents the opposite side of the author's argument is called a counterclaim.
Step-by-step explanation:
When we write an argumentative essay, the opinion we express is our claim or argument. We should explain why we have that opinion and defend it. What we defend it against are the counterclaims. A counterclaim is an idea or argument that is opposite to the claim.
For instance, imagine you are writing an essay claiming that a park should be built in your neighborhood. A possible counterclaim to that is the fact that parks may be dangerous at night, because people can use it to do or sell drugs. See how the counterclaim defends the opposite idea (that a park should not be built)?
With that in mind, we can complete the sentence as follows:
A line of reasoning that presents the opposite side of the author' argument is called a counterclaim.