“Las” is not the right answer. Spanish grammar is both beautiful and complex. It’s very important to take into account the fact that it’s formal language and it’s plural, but also, Spanish is a gendered language, which means everything is associated with a gender. “Las” is used with “female” or “feminine words and “los” with “male” or “masculine” words. Since cuchillo is a “male” word, you need to use “los”.
