The Tropics are the limits between which it is possible for the sun to be directly overhead at least once in a year - the kind of opposite of the Arctic and Antarctic Circles, which are the limits past which it is possible for the sun to not rise at all at least once per year.
Nice quiz, surprisingly high % guessed. I got all but one with a minute to spare. Didnt get it... ugh, southafrica. I tried lesotho, swaziland, zambia, zimbabwe even malawi.. Ah well I was happy to get all the others without trying wrong ones first.
I was flustered when the countries weren't in order. Then I realized they were going west to east. Then I read the description that already explained this :/