The code follows a pattern where each letter is shifted backward by its position in the word. For "DELHI":

The first letter "D" is shifted back by $1$, resulting in "C". The second letter "E" is shifted back by $2$, resulting in "C". The third letter "L" is shifted back by $3$, resulting in "I". The fourth letter "H" is shifted back by $4$, resulting in "D". The fifth letter "I" is shifted back by $5$, resulting in "D". Thus, the code for "DELHI" is "CCIDD."