What is actually a round seizing and is used wherever a temporary eye is needed on the middle of a line?

A round seizing is utilized to create a temporary eye in the middle of a line, and throat seizing is specifically known for this purpose. It is a technique where a line is wrapped around another line to secure it in position, forming a loop that can be used for various functions, such as attaching a hook or creating a temporary tie. This makes it particularly effective when a quick and reliable method of binding is needed.

The other choices, while also related to securing lines, serve different functions. Flat seizing, for instance, is generally used to bind two pieces of rope or a rope to a spar flat against one another. Frapping seizing is designed to bind the coils of rope together, usually for neatness or to prevent movement. Racking seizing might be related to securing an object but is not specifically intended for creating a temporary eye. Thus, throat seizing's specific purpose and technique make it the correct answer for the creation of a temporary eye in a line.