Shy and anxious children who have mild to moderate degrees of intellectual dysfunction, language impairments, and behavioral problems are MOST likely to be diagnosed with:

A) trisomy 21.
B) Down syndrome.
C) phenylketonuria.
D) fragile X syndrome.

Fragile X syndrome is characterized by a range of developmental problems including intellectual disabilities, anxiety, and behavioral issues. It is also associated with physical features and is the most common form of inherited intellectual disability. Trisomy 21 and Down syndrome refer to the same condition, which does not specifically emphasize mild to moderate intellectual dysfunction, language impairments, and behavioral problems in the context of the question. Phenylketonuria is a metabolic disorder that, if untreated, leads to intellectual disability but is not characterized by the specific combination of symptoms described in the question.