"A decade or a year"
Translation:Et årti eller et år
"En" is the indefinite article for the common gender, while "et" is the indefinite article for neuter gender. This was introduced way back in the first couple of lessons.
Duolingo doesn't always do the best job of making things like this clear contextually, but if you scroll down and read the text that accompanies each lesson before diving into the actual lessons, things like this are explained in way more detail, and make everything much less confusing.