Estamos: we are
estas: you are
estan: they are
The question says where are Maria and Adela? You should change “Maria and Adela” to they, do the third option is correct.
When you're talking about nonspecific things.
it's like (a) and (an) in english.
uno is for singular masculine.
una is for singular feminine.
unos is for plural masculine.
unas is for plural feminine.
The theory developed based on observations made on the Galapagos islands was, Natural Selection.
I hope I helped you!
Source: https://socratic.org/questions/which-theory-was-developed-based-on-observations-made-on-the-gal-pagos-islands
