The metal that can be cut with a knife is (b) sodium.
Explanation: Sodium is a soft metal from the alkali group, making it easily cuttable with a knife, unlike harder metals like iron or gold.
Why other options are incorrect:
(a) iron: Iron is a hard metal, and is not easily cut with a knife.
(c) gold: Gold is also a hard metal, with high density, making it difficult to cut with a knife.
(d) magnesium: While not as hard as iron or gold, magnesium is still relatively hard and not easily cuttable with a knife.