The correct answer is B, "this amendment will not solve the problem of sex discrimination".
Counterargument is an objection. A counterargument can be used to refute an objection to an argument prior to the argumentative conclusion, a main assertion or a slogan. Some synonyms of counterargument are replication, counterclaim, and counter response. The attempt to refute an argument can involve the generation of a counterargument or find a counterexample, that is, a reason that opposes another already proposed.
In this case, the Amendment proposed equality of rights and non-discrimination on the basis of gender in the US, so a counter-argument would be the fact that the Amendment would not guarantee equal rights, which is made explicit in statement B.