On the job training

This question is from Kaplan
You have been asked to comment on the most effective approach for training accounts staff in the use of new payroll system . Which of the following arguments would you put forward for choosing an on the job approach ?
1) ability of learner to concentrate on learning process
2) risk of errors
3) relevance to the informal customs and practices of the department
4) application of learned skills to job

I thought answer should be risk of error because in on the job training there is risk of error and application of learned skills to job is its advantage then why the answer is (4)in the kit ain’t 2 should be the argument that on job training will cause risk of error . Can anyone please clear my confusion