'Folha" has about 25 different meanings including leaf, foliage, petal, part of a door, blade, journal, list, report, register, newspaper, periodical, proof sheet, payroll. There several other meanings when it is followed by "de" such as folho de trabalho meaning time card. Feel free to choose anyone you like. With luck you might select one accepted by Duo. They certainly did'nt make a good choice that makes any sense.
Both paga and pagada are correct. Some verbs have both regular and irregular participles.
See this page for a discussion: http://redeglobo.globo.com/sp/tvtribuna/camera-educacao/platb/2013/09/18/ele-tinha-pago-ou-pagado-a-conta/
In portuguese you have one word for two different things uma folha de arvore = leaf , e uma folha de papel o de livro etc = sheet. so actually they are two different things but in portuguese you use one word but in english you have two and "leaves" is the irregular plural for "leaf" unless this is the way i have learned one day.
My question is not so much about the translation, which is fine with me, but about the use of passive voice in general in Portuguese; do Portuguese speakers and writers use the passive voice regularly? In English we are usually instructed to avoid the passive voice and i was wondering if the same is true in Portuguese. I am also doing Dutch with Duolingo and that language seems to use the passive voice a lot. How about Portuguese?