Final answer:
The correct answer is 'a) Autosome' because the loss of an autosome chromosome contains essential genes, leading to spontaneous abortion as development to birth is typically not possible.
Step-by-step explanation:
The loss of a/an autosome chromosome is often spontaneously aborted because life is usually not possible. Hence, the correct answer to the question is "a) Autosome". An individual with the proper number of chromosomes is referred to as euploid, which in humans means 22 pairs of autosomes and one pair of sex chromosomes.
In contrast, an aneuploid individual has an abnormal number of chromosomes, including conditions like monosomy (loss of one chromosome) or trisomy (gain of an extra chromosome). When a monosomic zygote is missing an autosome, it almost always fails to develop to birth because essential genes are missing. This demonstrates the critical importance of gene dosage in humans.
On the other hand, trisomies of certain smaller autosomes can result in offspring that may survive for a time, albeit with various challenges. The most common example of a trisomy that can result in a viable birth is Down syndrome, which involves an extra copy of chromosome 21.