The cutting technique used to layer very long hair and keep weight at the perimeter is:

The correct answer is slide cutting, which is a technique often employed to create layers in very long hair while maintaining weight at the perimeter. This technique involves using the scissors to slide along the hair strands, which allows the stylist to remove bulk and add texture without compromising the overall shape and weight of the ends.

By utilizing slide cutting, stylists can achieve a softer, more blended appearance in the layers, which is especially important for long hair where harsh lines can be unflattering. This method helps to ensure that while the internal structure is layered, the outer layer remains full and weighted, providing a polished look that maintains the integrity of the hair's length.

Other techniques, such as stacking or blunt cutting, do not achieve the same effect. Stacking techniques are generally used to create a more pronounced layering effect with increased volume, which would not serve the purpose of maintaining length at the perimeter. Scissors over comb is a technique typically used for short styles or for precision work close to the scalp, while blunt cutting results in a straight across finish without layering, which contradicts the need for layering in this scenario. Therefore, slide cutting is the most appropriate choice for layering long hair while keeping weight at the perimeter.