This is idomatic (basically a special case that you just have to memorize). In this case "seat" is basically "capitol" or "headquarters". These words can take "the", but it is not necessary.
Here is my TOEFL advice:
1) If you think an article is missing check the other three answers carefully first! TOEFL may be trying to trick you! I have never seen a TOEFL questions that asks you to choose between "a" or "the", and missing article questions are rare! There is usually an "a" or "an" question though!
2) Check out my sites below!