Questions & Answers about Hun har en grønn bok.
Norwegian has two indefinite articles:
• en for common-gender nouns
• et for neuter-gender nouns
Since bok is a common-gender noun, the correct article is en.
In Bokmål, there are two grammatical genders:
• Common (en-words): covers both masculine and feminine nouns
• Neuter (et-words): a separate class
Common-gender nouns use en (or optionally ei for feminine) in the singular indefinite.