Final answer:
The landmark Supreme Court case that would deny the prosecution the death penalty as a sanction for someone with a developmental disability is Atkins v. Virginia (2002).
Step-by-step explanation:
The landmark Supreme Court case that would deny the prosecution the death penalty as a sanction for someone with a developmental disability is Atkins v. Virginia (2002). The case involved Daryl Atkins, who had an IQ of 59, and the court ruled that executing individuals with intellectual disabilities is considered 'cruel and unusual punishment' prohibited by the Eighth Amendment.