A. Consistent failure to speak in specific social situations in which there is an expectation for speaking (e.g., at school) despite speaking in other situations.
B. The disturbance interferes with educational or occupational achievement or with social communication.
C. The duration of the disturbance is at least 1 month (not limited to the first month of school)
D. The failure to speak is not attributable to a lack of knowledge of, or comfort with, the spoken language required in the social situation.
E. The disturbance is not better explained by a communication disorder (e.g., childhood-onset fluency disorder) and does not occur exclusively during the course of autism spectrum disorder, schizophrenia, or another psychotic disorder.And that's that. The disorder is most common in children five and under. Knowing the above criteria should allow you to easily identify the correct answer in any social work licensing exam on the topic!
For more reading about selective mutism, try selectivemutismcenter.org, AnxietyBC, & Medline Plus.