The crystal field stabilization energy (CFSE) and magnetic (spin-only) of an octahedral aqua complex of a metal ion \( (M^{z+}) \) are \( -0.8\,\Delta_0 \) and \( 3.87\,\text{BM} \), respectively. Identify \( (M^{Z+}) \):
\( \mathrm{Mn^{4+}} \)
\( \mathrm{Co^{2+}} \)
\( \mathrm{V^{3+}} \)
\( \mathrm{Cr^{3+}} \)