Answer: Amicus curiae
Explanation: Amicus curiae is described as the party that is not the part of the case but can assist the court in the form of "friend of the court" by presenting the information and advice on any particular case. The consideration on the information given by amicus curiae depends on judgement of the court.
As per the case mentioned in the question ,National Rifle Association is acting as the amicus curiae because they are sending the informative brief to the court for reversing the conviction.