Final answer:
The sex chromosome combination YO male is not viable and has never been observed in a human birth, unlike XO female, XXY male, and XXX female which have all been documented.
Step-by-step explanation:
The correct answer to which sex chromosome combination has never been observed in a viable human birth is A. YO male. In humans, the presence of at least one X chromosome is crucial for embryonic development. Individuals with the genotype XO (Turner syndrome) are female with certain physical anomalies and sterility, those with XXY chromosomes (Klinefelter syndrome) are male with characteristic physical features and often reduced fertility, and individuals with XXX chromosomes (triplo-X) are female with possible developmental delays and reduced fertility. Without any X chromosomes, the embryo lacks vital genetic information necessary for survival, thus a YO male is not viable and has not been observed.