The sentence "Nous avons __ voiture rouge" requires an article that agrees with the noun voiture (car). In French, voiture is feminine singular, so the correct indefinite article is une: "Nous avons une voiture rouge" - "We have a red car."
The other options do not fit:
des is the plural indefinite article ("some"). It would require a plural noun: "des voitures rouges" (some red cars). The noun here is singular.
un is the masculine singular indefinite article. It is used with masculine nouns (un livre, un stylo), but voiture is feminine.
le is the masculine singular definite article. It fails on two counts: wrong gender (masculine instead of feminine) and wrong type (definite instead of indefinite, changing the meaning to "the" rather than "a").
To choose the correct article, always identify both the gender (masculine or feminine) and the number (singular or plural) of the noun it accompanies.
