In the hexadecimal system, each digit represents a value from 0 to 15. When we reach a value of 10, we use letters to represent the remaining values until 15. The letters used are A, B, C, D, E, and F, which correspond to the decimal values 10, 11, 12, 13, 14, and 15, respectively.
Therefore, in the hexadecimal system, D represents the decimal value 13.