The names end in al.
The "al" is from the word "aldehyde".

There are four carbons, hence butan.
The =O is present at the end of the chain.
These compounds are called aldehydes and their names end in al.
This group can only be at the end of a chain, hence no number is required.

A chlorine is at carbon 2.

Hence the name is:
2-chlorobutanal