I'll list the most natural answers according to what I would say, and try to explain why if possible. 1. "Have you ever been to Croatia?" seems the most natural to me, although in certain contexts "Were you ever in Croatia?" could also be appropriate. 2. "Why don't you take an aspirin, then?" Would be the most appropriate answer in this case because it is offering a suggestion to make the first speaker's headache go away. 3. I honestly don't know which answer would be expected in this case, sorry. 4. Again I'm not certain which answer would be best, I would use the word socket, but that is not an available choice... 5. Perhaps "unusual" would be correct here, if a desk is made of luggage I would probably consider it quite strange, unusual, or abnormal. 6. The correct choice here is "to send" because you already have a verb "forgotten" and now you need the infinitive. So "I've forgotten to send Aunt Mary a birthday card." is the correct sentence. 7. In this case the correct word depends on context I suppose complicated and elaborate both seem appropriate, but I would say the correct word is probably "elaborate". I hope that was helpful for you!