A denatured protein Your answer: has undergone a process which changes the D isomer to an L isome

"Denaturation" refers to the process by which a protein molecule loses its shape. When this happens, it becomes unable to carry out its function (usually leading to a critical failure). Therefore, the answer is "does not have the function it previously had". For the record, the process which changes a D isomer to an L isomer (flipping the configuration of an enantiomer) is "inversion", whereas "stereoisomer of the original form" can refer to this or it can refer to changing a molecule into a diastereomer - which is called "epimerization".