In which key does the answer usually enter during a fugue exposition?

In a fugue, the answer typically enters in the key of the dominant, which is the fifth degree of the scale.1

During the exposition, the subject is first stated alone in the main key. When the second voice enters with the answer, it is transposed to this dominant key. If the answer is an exact transposition of the subject, it is referred to as a real answer; if the melody is slightly adjusted to maintain the tonal center, it is known as a tonal answer.1