Answer:
C) By creating a friendly classroom environment that is supportive and enabling
Explanation:
According to Stephen Krashen's theory of second language acquisition, a teacher can keep learners' 'affective filter' in check by creating a friendly classroom environment that is supportive and enabling. Second language acquisition does not require according to Krashen's theory a tedious drill of grammatical rules. It is important to have a meaningful interaction in the target language creating a friendly atmosphere in the classroom.