Final answer:
In prophase I of meiosis, an organism with a diploid number of 2n = 18 would have 36 sister chromatids present, as each of the 18 chromosomes consists of two sister chromatids.
Step-by-step explanation:
If 2n = 18, this implies that the organism is diploid and has 9 pairs of homologous chromosomes, where 'n' represents the haploid number. During prophase I of meiosis, each chromosome has already replicated into two sister chromatids. However, at this stage, they are still joined together at the centromere, appearing as one unit under a microscope.
Since 2n = 18 refers to the number of chromatids in a diploid cell, we have 18 chromosomes each consisting of two attached sister chromatids. This means that in prophase I, there are a total of 18 x 2 = 36 sister chromatids present. Therefore, the correct answer to the question is a. 36.