A nurse is educating a group of parents about potential causes of autism spectrum disorder (ASD). Which of the following responses should the nurse make?

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

"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."