How to form a relative clause with a 是 in it?
Unlike in English, in Chinese, an attributive clause is not long in general. Instead, we prefer to a subject-detail stucture over the sentence-attributive cluase structure.

In your case, if you insist on an attributive, you can say


  我选的是考试（形式）是谈话的课程
  What I take is classes whose tests (forms) are conversations".


This is correct, however, more naturally, I would say:


  我选的课程，其(它的)考试形式是谈话。
  The classes I take, whose (their) test forms are conversations.


Here, you first propose a topic/subject - 我选的课程, and then you start explaining its details. This is how Chinese language organizes the sentence structure.

Notes: 


I think the verb "take" in your case means "to choose/pick/select/participate in". The verb "采取" (to adopt/use/take) generally precedes nouns like 方法(method)，措施(measure),方案(plan).
I think "考试形式(test forms)" sounds more natural.
pay attention to the word "其"，it's a formal word used like "which/whose" as in an attributive clause in English. You can use it as a pronoun in the cluase to avoid mentioning the subject repeatly.