I've learned that it's the second-largest, so I'm pretty sure that was the meaning of it. But yeah, I also hate it when questions are imprecise and ambiguous.
Instead of assuming others are imprecise first seek to understand whether there's something you are unaware of. It would have saved you from embarrassing yourself.
This is actually a fairly common geographic term. From Wikipedia: A second city is the second-most important city in the urban hierarchy of a country or region, often in contrast to a primate city.
Lake Chad is in either central or west Africa. Next to Benin you can find Nigeria, not just Togo. In Nigeria Hausa and Yoruba are as well one of the most common languages yet you do not accept them as correct answer.
Lake Chad is not in West Africa bud. Considering the size of Africa, it's pretty far North. So I don't know what your logic is. Central Africa is not very specific for a continent like Africa.