If Mario completed an intense core training workout with correct form, he would most logically feel soreness in his abdominal muscles, especially in the rectus abdominis, obliques, and transverse abdominis. Core workouts typically engage these muscles significantly, leading to soreness in that area.
While some core exercises can involve the hip flexors and even the lower back, soreness in the legs, neck, or arms is less common unless those areas were specifically targeted during the workout. Therefore, the primary area of soreness would likely be in his abdomen.