The correct answer is:
C) that is
Explanation:
In the given sentence, the phrase "which is" introduces additional information about the brain stem. This type of phrase is called a relative clause, and it should be set off by commas if the information provided is not essential to the meaning of the sentence. In this case, the information that the brain stem is the oldest part of the brain in evolutionary terms is essential to understanding which part of the brain is being discussed. Therefore, the correct choice to maintain the sentence's meaning and structure is "that is."
