Which of the following tools is essential for achieving a clean line in a haircut?

The correct choice is clippers because they are specifically designed to provide precise cuts and can be used to create clean lines in a haircut. Clippers come with various guard sizes that allow for different lengths, and they can easily contour the hair on different areas of the head, making them an essential tool for barbers aiming for sharp, defined edges. When used properly, clippers can also help achieve a seamless blend between shorter and longer sections of hair, enhancing the overall look of the haircut.

While the other tools mentioned serve important purposes, they do not specifically focus on creating clean lines. Combs are primarily used to lift and section hair for cutting, texturizing shears are meant for adding texture and removing bulk without altering the overall length drastically, and razors are typically used for softer finishes and blending but are not the primary tool for achieving distinct, clean lines.