To answer this question, we need to understand the meaning of the word "turncoat".
Option A) informal discussion - This option is incorrect. "Turncoat" does not refer to an informal discussion. It is a noun that describes a person who switches allegiance or loyalty, especially to the opposing side.
Option B) circular building or hall covered with a dome - This option is incorrect. "Turncoat" does not refer to a circular building or hall covered with a dome.
Option C) discourage - This option is incorrect. "Turncoat" does not mean to discourage. It is a noun that describes a person who is a traitor or switches sides.
Option D) traitor - This option is correct. "Turncoat" is a noun that refers to a traitor, someone who switches sides or betrays their allegiance.
Therefore, the correct answer is D) traitor.