Learn Spanish (Latin America): Lo que ustedes ofrecen sin la garantía es inaceptable. - What you offer without the warranty is unacceptable.

Qué with an accent is a question word meaning "what" (as in ¿Qué ofrecen? - "What are you offering?"). Here, "what" isn't a question; it's a statement meaning "the thing that." To say "the thing that" or refer to an abstract idea, Spanish uses the neuter relative pronoun lo que.

What is the subject of the verb es in this sentence?

The subject of es is the entire first phrase: Lo que ustedes ofrecen sin la garantía. Because lo que represents an abstract idea, the entire clause acts as a single, abstract subject, which takes the singular verb es.

Since lo que is a neuter pronoun, how do we handle the gender of the adjective inaceptable?

When an adjective describes an abstract concept or a lo que clause, it always defaults to the masculine singular form. The adjective inaceptable ends in an "-e", so its masculine and feminine forms are identical anyway. However, if you used an adjective with gender variations, like malo, it would take the masculine form: Lo que ofrecen es malo.

Can I leave out ustedes and just say Lo que ofrecen...?