The error that Dan made was that D. A one-sample test for a proportion should have been performed because only one sample was used.
Dan should however have only used a one-sample test because the he only used one sample. By using a test that catered for two proportions, he misanalysed the data.
In conclusion, option D is correct.
Find out more on sample tests at https://brainly.com/question/15683511.