Answer: The correct answer is- 38.
Haploid is a state of cell that represents single/ one set of chromosomes (depicted by n, which indicates number of chromosome in one basic set) in the cell whereas diploid (depicted by 2n) represents two basic sets of chromosome in the cell.
In this question, haploid number of sphagnum moss is 38.
Thus, it's diploid number would be-
2×19 = 38