Correct answer:
costas
Costas is a feminine plural noun. Recall that a noun is a word or group of words that is the name of a person, a place, a thing, or activity or a quality or idea. So costas means shores and is the land that is on the edge of an ocean, lake or river. What this sentence means is that I take the sun on the shores of the Rio de la Plata. Finally:
Yo tomo el sol en las costas del Río de la Plata.
