Air conditioning is an optional accessory.

Literal

Air-conditioner [topic-は] option [gen-の] accessory [copula-です].

エアコン is a wasei-eigo abbreviation from 'air conditioner.' 付属品 'accessory / included item' (付属 'attached to' + 品 'item'). A minimal, factual product-specification sentence with no grammatical complexity.