Final answer:
The correct definite article to use in the phrase 'reglas rojas para la clase de geometría' is 'las'. The noun 'reglas' is plural and feminine, so we use the feminine plural definite article 'las'. The correct phrase is therefore 'las reglas rojas para la clase de geometría'.
Step-by-step explanation:
The correct definite article to use in the phrase 'reglas rojas para la clase de geometría' is las.
The noun 'reglas' is plural and feminine, so we use the feminine plural definite article 'las'.
The correct phrase is therefore 'las reglas rojas para la clase de geometría'.
Learn more about Definite articles in Spanish