Think about what happens physically when the indenter presses into a hard sample. Ball shaped indenters, whether steel or tungsten, can flatten or wear when pushed into very hard metals, so Brinell and some Rockwell scales rely on that ball design. Vickers avoids this problem completely by using a square based diamond pyramid, since diamond will not deform under load no matter how hard the test piece is. The pyramid leaves a neat square impression on the surface, and you simply measure the two diagonals under a microscope to get the hardness number. A diamond cone is used in Rockwell C, not Vickers. So the indenter in Vickers testing is the diamond pyramid, option (C).