Final answer:
The haploid number is half of the diploid number. For an organism with a diploid number of 90, the haploid number is 45.
Step-by-step explanation:
If an organism has a diploid number of 90 chromosomes, then its haploid number is half of the diploid number. Haploid cells, such as gametes, have one set of chromosomes, which equates to half the number found in diploid cells. Diploid cells have two sets of chromosomes, hence the term 2n, where n represents the haploid number of chromosomes. Therefore, the correct answer to the question of a diploid number being 90, making the haploid number, is C. 45.