Hey there! I'm happy to help!
Let's look at our vocabulary words here.
romper- to break
el brazo- the arm
nadar- to swim (similar to the word nautical, navy, etc.)
el río- the river (like Café Río!)
las cascadas- the waterfalls (similar to word cascades)
las llanuras- the plains
las montañas- the mountains (this is where the state Montana comes from)
las tierras- the lands (similar to words like terrain)
We see that Estela broke her arm swimming. Well, the only thing that has to do with water that is an answer option is the waterfalls, so the answer is las cascadas.
Have a wonderful day! :D