You can backsolve this one. Start with (C): If there are 8 girls on the bus, that means that there were 8 boys on the bus to start as well.

At the first stop, 4 boys got off and nobody got on, so now there are 4 boys on the bus and still 8 girls.

That's twice as many girls as there are boys (just like the question says) so (C) is our answer.