Step 1: Read the IUPAC name.
The compound is 2-Butanone. The ending one tells us it is a ketone, and but tells us it has four carbon atoms in the chain. The number $2$ shows the carbonyl group sits on the second carbon.
Step 2: Draw the structure.
Placing the C=O on the second carbon of a four carbon chain gives \[ CH_3-CO-CH_2-CH_3 \] This is the structure of 2-butanone.
Step 3: Look at both sides of the carbonyl.
A ketone has the carbonyl carbon joined to two carbon groups. Here one side is $CH_3$ and the other side is $CH_2CH_3$.
Step 4: Name the two groups.
The $CH_3$ group is called methyl, and the $CH_2CH_3$ group is called ethyl. Recognising these two groups is the heart of the common name.
Step 5: Build the common name.
For ketones, the common name lists the two attached groups in alphabetical order, then adds the word ketone. Ethyl comes before methyl, giving ethyl methyl ketone.
Step 6: Pick the correct option.
So the common name of 2-Butanone is Ethyl Methyl Ketone, which is option 4. Acetone is the common name of propanone, so it is a trap. \[ \boxed{\text{Ethyl Methyl Ketone}} \]