Based on the options provided, the unusual characteristics of the human karyotype can be assessed as follows:
a) 14 has enlarged centromeres: Enlarged centromeres can occur but may not be considered a significant abnormality in the context of a karyotype.
b) 21 has an extra chromosome: This describes Down syndrome (Trisomy 21), which is a well-known condition caused by the presence of an extra copy of chromosome 21.
c) 5 has chromosomes of different shapes: Variations in chromosome morphology can occur, but significant structural abnormalities would typically be more relevant.
d) 10 is missing genetic material: A missing part of a chromosome can indicate a deletion, which may lead to various genetic disorders.
Among these, option b (21 has an extra chromosome) is the most notable and recognized chromosomal abnormality, as it corresponds to a specific genetic condition. Thus, the unusual aspect of the karyotype, in this case, is the presence of an extra chromosome 21.