Magnesium, though not easily, can be cut with a knife.
Sodium and Potassium are very soft metal can be very easily cut with a knife, just like wax.
These metals have large size and only 1 electron in their valence shell, due to this the atomic bonding is quite weak
Also, these metals have body centred cubic structure that has a lot of empty space and this makes metallic bond weak.
Hence, these can be easily cut using a knife
Calcium is an alkaline earth metal. It can also be easily cut with a knife.
Magnesium is a bit hard as it has a small size and 2 electrons in the valence shell. So, it has comparatively better atomic bonding than others given in the options.