Which word is different in terms of prefix meaning?

Prefixes are word parts added to the beginning of a word to change its meaning. Many prefixes convey negation, while others indicate time, place, or other relationships.

• D: Postpone. This is the correct answer. The prefix "post-" means "after" or "later." Therefore, "postpone" means to put off until a later time. Its meaning is related to time, not negation.
• A: Unhappy. The prefix "un-" means "not" or "the opposite of." So, "unhappy" means not happy.
• B: Inactive. The prefix "in-" (a variant of which can be "im-", "il-", "ir-") means "not" or "without." Thus, "inactive" means not active.
• C: Illegal. The prefix "il-" is a variant of "in-" used before words starting with 'l'. It also means "not." So, "illegal" means not legal.

All the other options use prefixes that convey a sense of negation or opposition, making "Postpone" the word with a different prefix meaning.