Final answer:
The true statements are that a monosomic individual would have 9 chromosomes and a double trisomic individual would have 12 chromosomes, since the normal diploid number is 10.
Step-by-step explanation:
Considering a eukaryotic organism with a normal diploid number of 10 chromosomes, we can analyze the statements provided. A monosomic individual is one that has lost a chromosome and therefore would have one chromosome less than the diploid number, resulting in 9 chromosomes (10-1=9).
On the other hand, a double trisomic individual has gained two extra chromosomes, so they would have a total of 12 chromosomes (10+2=12). Therefore, the correct answer is c. Both A and B.