Which surveying device operates only on a flat surface?

The level is specifically designed to operate on flat or level surfaces, making it the correct choice for this question. This instrument is utilized to establish a horizontal plane, which is essential in surveying, construction, and various engineering applications. The level works by determining equal elevation points across a distance, and its accuracy relies on the instrument being positioned on a stable, flat surface to ensure precise readings.

While the transit, plane table, and theodolite are valuable surveying instruments, they are not limited to flat surfaces. For instance, the transit and theodolite can be used on uneven terrain and are capable of measuring angles both horizontally and vertically. The plane table is versatile and can be set up on sloped or uneven ground to create maps and plans. However, the level's primary function hinges on its ability to obtain horizontal lines, restrictively adapting it to flat surfaces for optimal performance.