Which of the following tools is not suitable for etching?

The best choice for the tool that is not suitable for etching is files. Etching is a process typically used to remove material from a surface for detailed designs or patterns, often utilizing acids or other solvents, rather than mechanical abrasion. Files are designed for smoothing and shaping edges but do not facilitate the precise control needed for etching processes.

In contrast, locking pliers, duckbill pliers, and needle nose pliers are all tools that can assist in manipulating materials during etching or other related tasks, such as holding or positioning. Their design allows for grip and control that could be beneficial in various metalworking procedures, including etching. Therefore, files are not appropriate for the etching process due to their intended use and functional characteristics.