Proteins are large molecules. They enter cells through endocytosis.
Hence the correct answer is diffusion.
Proteins and other large molecules can not enter the cells through the cell membrane.
For the entry they are the first wrap into the cell membrane and the engulfed by the cell and process is known as endocytosis.
As this is diffusion because proteins are moving from high concentration to low concentration. But they are facilitated by the cell membrane. So it will be facilitated diffusion.
