Explanation: Here we need to complete the sentence:
Soy alumno en ____________________ escuela secundaria americana.
and the options are:
A. Un
c. Unos
b. Una
d. Unas
The subject here is "escuela secundaria americana", and is easy to see that is feminine and singular.
Then, the right option is b: una, because it is also in the feminine (has an "a" in the last syllable) and is in the singular (it does not end with an s)