Step 1: Recall the definition. An antonym is a word that has the opposite meaning. Step 2: Identify the meaning of 'ancient'. 'Ancient' means very old, from a long time ago. Step 3: Test each option for opposite meaning. 'Old' = very old → same meaning as 'ancient' (synonym, not antonym). 'Modern' = new, from recent times → opposite of ancient (antonym). 'Historic' = important in history → related but not opposite. 'Aged' = very old → same meaning as 'ancient' (synonym). Step 4: Select the answer. 'Modern' is the opposite of 'ancient'. Answer: modern