Question

whats the main difference between \otro\ and \otra\?
there is absolutely no difference.
\otro\ is masculine and \otra\ is feminine. they change depending on the gender of the noun.
both mean \another\, but \otro\ is used by females and \otra\ by males.

Explanation:

In Spanish, "otro" is masculine and "otra" is feminine, used according to noun - gender.

Answer:

B. "Otro" is masculine and "otra" is feminine. They change depending on the gender of the noun.