The names end in one.
The "one" is from the word "ketone" and is pronounced as "own".

There are four carbons, hence butan.
The =O is present at the second carbon.
These compounds are called ketones and their names end in one.
Hence 2-one
This group must only be at an in-between carbon to be a ketone.

A chlorine is at carbon 3.

Hence the name is:
3-chloro-butan-2-one