How did you get on? I had thought about giving you the same options I gave my students, but changed my mind on the grounds it was too easy to whittle it down to the right answer.
If you said lighthouse as some of my students did, - emmm - nope. The 1st lighthouse was built in 280 BC - Pharos lighthouse of Alexandria (one of the 7 wonders of the world).
In 1822 French Physicist, Augustine Fresnel invented a lens that would make his name commonplace in the maritime world. This lens has 1,176 prisms and 24 bullseyes. At the centre, the bullseyes work like a magnifying glass so that the light beam is even more powerful. The lens sections are held together by brass frames weighing five to six tons. The entire lens weighs in at approximately six to eight tons casting its light out to a distance up to 28 Nm.
There is a very good article in the Britannica if you are interested.