What is the IUPAC name of the compound CH3−CH=CH−CH3?

The compound contains a carbon-carbon double bond ($=CH-CH=$), which is characteristic of an alkene.

Count all the carbon atoms in the main chain that includes the double bond.

There are four carbon atoms in the chain. The parent alkane with four carbon atoms is butane. Since it contains a double bond, the suffix changes from '-ane' to '-ene', making it butene.

If we number from left to right, the double bond starts at carbon 2.
If we number from right to left, the double bond also starts at carbon 2.
So, the position of the double bond is 2.

Combine the position of the double bond with the parent alkene name.
The name is 2-butene.

Comparing this with the given options:
A. 1-butene (This would be $CH_2=CH-CH_2-CH_3$)
B. 2-butene
C. butane (This would be an alkane, $CH_3-CH_2-CH_2-CH_3$)
D. 1-butyne (This would be an alkyne, $CH \equiv C-CH_2-CH_3$)

The correct option is B.

The name of the compound is $\boxed{\text{B. 2-butene}}$.