Based on the statements provided about the human karyotype, the correct interpretation of "unusual chromosome set" would most likely correspond to:
c) 21 has an extra chromosome.
This refers to Down syndrome (Trisomy 21), where there are three copies of chromosome 21 instead of the usual two, leading to various developmental and physical characteristics. The other options either describe anomalies that are less commonly associated with specific syndromes or do not typically describe karyotype abnormalities in a straightforward manner.