I sold a car, _ plant and a European bike

Explanation

n oxygen, but only if we assume that "oxygen" is being used as a noun to refer to a plant that produces oxygen, such as an oxygen plant or an oxygen-producing plant.In this case, the sentence would read:"I sold a car, an oxygen plant, and a European bike."Here, "an" is the correct article to use before "oxygen" because it starts with a vowel sound.