The electronic configuration of Cr is [Ar]3d54s1 and that of Cu is [Ar]3d104s1. Their second ionisation enthalpies are higher than expected because after removal of one electron chromium gets a half-filled electronic configuration and Cu get a fully-filled 3d electronic configuration. These configuration are more stable than the partial-filled electronic configuration. So, it is very difficult to remove the second electron in both cases. Thus, option (d) is correct.