In the hexadecimal system, the digits used include letters from the alphabet. How many letters are included?

In the hexadecimal system, which is a base-16 numeral system, the digits range from 0 to 9 and include letters A to F. The letters represent the values of 10 to 15, where A stands for 10, B for 11, C for 12, D for 13, E for 14, and F for 15. Therefore, a total of six letters are used in the hexadecimal system. This makes the option stating that there are six letters the correct choice. The inclusion of these letters allows for a comprehensive representation of values beyond the decimal system, enabling efficient notation and calculations in computer science and digital electronics.