A nurse is reinforcing teaching to a group of parents about potential causes of autism spectrum disorder (ASD). Which of the following statements should the nurse reinforce?

“Vaccinations, especially the MMR vaccine, have been proven to be a leading cause of autism in children."

"ASD is primarily caused by poor parenting and a lack of nurturing in early childhood."

“The exact causes of autism spectrum disorder are not fully understood, but it is believed to be related to a combination of genetic and environmental factors."

"ASD can be caused by consuming gluten and casein, which are found in wheat and dairy products, respectively.”