Interpret the structure of compound $(X)$

From the options, the compound is clearly a bromo-substituted methyl alkene. We must assign the correct IUPAC name by following these rules:

• Choose the longest chain containing the double bond.
• Number the chain so that the double bond gets the lowest possible locant.
• Then assign substituent positions.

Select the parent chain

The longest chain containing the double bond has 4 carbon atoms.

So the parent hydrocarbon is but-2-ene or but-1-ene, depending on numbering.

Number the chain correctly

The double bond must get the lowest locant.

If the structure is numbered from the end nearer the double bond, the double bond comes at carbon $2$.

Now compare the substituent positions:

• One substituent is bromo at carbon $1$
• One substituent is methyl at carbon $2$

Hence the name becomes:

$\boxed{\text{1-Bromo-2-methylbut-2-ene}}$

Check the options

• A: 4-Bromo-3-methylbut-2-ene
  Incorrect numbering; substituents should get lower locants after fixing the double bond locant.

• B: 1-Bromo-2-methylbut-2-ene
  Correct.

• C: 3-Bromo-3-methylprop-2-ene
  Incorrect parent chain chosen; not the proper longest chain.

• D: 2-Bromo-2-methylbut-2-ene
  Incorrect substituent position for bromine.

Final answer

Therefore, the correct IUPAC name is:

$\boxed{\text{1-Bromo-2-methylbut-2-ene}}$