"Raj has written a book."
Translation:राज ने एक किताब लिखी है।
I was always under the impression that the indefinite article was unnecessary in Hindi unless specifying a number or putting the object in contrast with another; why was 'राज ने किताब लिखी है' marked wrong?
You were marked wrong because you haven't specified the quantity as to how many books raj has written. In the above sentence, it was mentioned "a book" which means "one book" in Hindi language.