To determine the odd item from the given alternatives, we need to examine the pattern or logic governing the grouping of letters in each option: acDD, egHH, ikLL, and QSTt.

• acDD: The first two letters 'a' and 'c' are sequential in the English alphabet. The last two letters 'D' and 'D' are identical uppercase letters.

• egHH: The first two letters 'e' and 'g' have a sequence pattern similar to 'ac' (with one letter skipped between them). The last two letters 'H' and 'H' are identical uppercase letters.

• ikLL: The first two letters 'i' and 'k' follow the same pattern as the previous pairs. The last two letters 'L' and 'L' are identical uppercase letters.

• QSTt: The first two letters 'Q' and 'S' are not sequential (missing letter), and the addition of 't' as a lowercase letter disrupts the pattern of two identical uppercase letters at the end.

The pattern in the first three options shows two sequential lowercase letters followed by two identical uppercase letters. The option QSTt breaks this pattern, making it the odd item.

The correct answer is: QSTt