Solution:(d) The rider who met with the accident was not wearing a helmet, the police said. (Correct)(a) The rider who met with a accident was not wearing a helmet, the police said. (Incorrect Article)
(b) The rider who met with an accident was not wearing an helmet, the police said. (Incorrect Article)
(c) A rider who met with the accident was not wearing the helmet, an police said. (Incorrect Article)