Which tool is essential for creating straight lines in nail art?

Striping tape is essential for creating straight lines in nail art because it serves as a guide that allows the artist to achieve clean and precise lines. When applying nail polish or design elements, striping tape can be placed on the nail to mask areas where color should not go. This results in a crisp demarcation that showcases well-defined shapes and patterns without the risk of smudging or uneven edges.

The other tools listed serve different purposes in nail design and do not focus specifically on line creation. A nail brush is primarily used for painting or applying color to the nails, while a dotting tool is meant for creating dots and intricate details. A nail file is used for shaping and smoothing the edges of the nail, but it does not contribute to the application or design aspects of nail art. Thus, striping tape stands out as the most relevant and effective tool for achieving straight lines in nail art.