Soy alumno en escuela secundaria americana.
a. un
c. unos
b. una
d. unas

It's una because escuela is a feminine noun

Los exploradores quieren  descubrir  nuevos mundos.

There are many examples of how latin influence has come to america and is here to stay. one for example is the banana. a fruit that was only grown in south america for years but then came to america in the early 1900s. it was propagandized as an exotic fruit from latin america during the first world war and took america by storm. now bananas are wildly used in the united states as one of the most popular fruits. another example is clothing. now more than ever ponchos, tribal prints from latin american latinos (such as mexico, peru, honduras, colombia, etc) are popularizing the colorful prints the latin american indigenous people would use, on back packs, sweaters, jewelry, blankets, etc in the "hipster movement." these things were originally popularized in the 90s after a mass movement of immigration after the regan administrations amnesty. but this trend continues in america today.