What process is used to separate DNA fragments by size?

Gel electrophoresis is the process used to separate DNA fragments by size, making it the correct answer. This technique involves applying an electric current to a gel matrix, where the DNA samples have been loaded. Since DNA is negatively charged due to its phosphate backbone, it migrates towards the positive electrode when the current is applied. Smaller DNA fragments move through the gel more easily and travel faster than larger fragments, resulting in the separation of the fragments based on size. After the process, the different sized DNA fragments can be visualized, allowing for analysis.

The other options are distinct processes: PCR (polymerase chain reaction) is primarily used for amplifying small segments of DNA, Southern blotting involves transferring DNA from a gel to a membrane for further analysis, and cloning refers to producing identical copies of an organism or cell. None of these techniques are specifically designed for the separation of DNA fragments based on size as gel electrophoresis is.