If an organism is a diploid (or 2n) with 16 chromosomes, then how many chromosomes its sperm cells or egg cells will contain?

The correct option is A 8
An organism consists of two types of cells in their body, somatic cells and germs cells/ gametes. During the process of meiosis, a diploid number of chromosome is cut down to haploid. This division is an important event since after fertilisation haploid chromosomes from each gamete (male and female) fuse to retain the diploid state in the zygote. Diploid means two sets of chromosome and haploid is one set. Since here the diploid set is 16 thus one set will be half it, i.e, 8. Thus the answer A is correct.