Metal X is soft and can be cut with a knife. It reacts vigorously with air and water. Identify metal X- iron
copper
Sodium
gold

The metal is most likely Sodium.

Explanation:

Iron is not soft enough to be cut with a knife and will not react violently with water.  When Iron reacts with water it just slowly rusts.

Copper is somewhat malleable, however, does not really react with water and only forms an oxide layer in the air.

Sodium is soft enough to be cut with a knife (I know this because I have worked with it and seen people work with it).  It is also an alkali metal which are known to explode when in water.

Gold is malleable but does not really react with anything and is considered inert.

it is the most commonly accepted one