The correct option is "enseña". The complete sentence would be "Ellas enseña inglés." This means "They teach English." Here, "ellas" (they) is a plural subject that would match with "enseñan" if you were referring to teaching in plural. However, if it's about a singular subject (ella), the sentence would read "Ella enseña inglés."
But since the subject is "ellas" (they), the complete sentence would correctly use "enseñan".
So the best choice is "enseñan".