What has a string made of metal wire platic or gut that are stretched across the bottom instrument?

The term you are looking for is " strings". Strings are an essential part of many instruments, including guitars, violins, cellos, and harps. They are made of various materials, including metal wire, plastic, or gut, and are stretched across the bottom of the instrument. When the strings are plucked, bowed, or strummed, they vibrate, producing sound. The type of material used for the strings, as well as the tension and thickness of the strings, affects the sound produced.