The correct answer is option 3: 1 and 4.
Option 1 states that `Milan` is an annual gathering of navies of Indian Ocean countries. This is correct. Milan is indeed a biennial multilateral naval exercise hosted by the Indian Navy. It brings together navies from the Indian Ocean region to strengthen their professional relationships, enhance mutual understanding, and promote maritime cooperation.
Option 2 states that `Milan` is conducted at Chennai. This statement is not mentioned in the question and therefore cannot be confirmed as correct or incorrect.
Option 3 states that `Milan` is a joint anti-terrorism exercise. This statement is incorrect. Milan is primarily focused on enhancing inter-operability among navies and fostering goodwill, rather than being specifically centered around anti-terrorism activities.
Option 4 states that `Milan` is aimed at enhancing inter-operability among navies. This statement is correct. As mentioned earlier, Milan aims to enhance cooperation and inter-operability among navies of the Indian Ocean region through a series of professional interactions, exercises, and cultural events.
In conclusion, the correct answer is option 3: 1 and 4. However, option 4 incorrectly includes the statement about `Milan` being a joint anti-terrorism exercise.