Question

he says it is one of the worst accidents that has occurred this year .
Spot the error

Answer 1

Answer:

I think it as 'Has'

I could have been 'had'

Answer 2

Answer:

He said that it was one of the worst accident that occurred this year.

Explanation:

The action has been performed so was is used .

There occured a single accident , so singular form must be used.