Final answer:
Hypotheses about chromosomes 21 and 18 suggest that chromosome 21 is smaller and carries fewer critical genes for early development, allowing individuals with trisomy 21 to survive into adulthood more often than those with trisomy 18.
Step-by-step explanation:
Based on the information about aneuploidies and the differential survival rates of individuals with trisomy 21 (Down syndrome) compared to those with trisomy 18, we can make certain inferences about chromosomes 21 and 18. The survival of individuals with trisomy 21 to adulthood indicates that while there is an excess in gene dose, the overall impact on essential developmental processes is less severe than in other trisomies. We can hypothesize that this is because chromosome 21 is one of the smaller human chromosomes and may carry fewer genes that are critical to early development.
Therefore, the more likely hypotheses are that chromosome 21 is smaller than chromosome 18 (option a), which can be inferred because trisomies involving smaller chromosomes are more likely to produce viable offspring, or chromosome 21 is less vital for survival than chromosome 18 (option b), as its triplication still allows for survival into adulthood, albeit with developmental challenges.
Chromosome 18 likely carries a higher number of or more critical genes needed for development, which means that the additional genetic material in trisomy 18 leads to more severe developmental problems that often preclude survival. Therefore, options (c) and (d) would be less supported given that chromosome 21 does carry crucial genes for survival, and chromosome 18's involvement in cellular processes is likely quite complex as well.