Step 1: What the rule needs:
The alkene must be unsymmetrical, so that two different products are possible and one is preferred.
Step 2: Test symmetry:
Draw a mirror between the double bond carbons. But-2-ene, hex-3-ene and ethene all have identical groups on each side, so the product is the same either way.
Step 3: Test 2-methylpropene:
It has $(CH_3)_2C$ on one end and $CH_2$ on the other, which are different. The proton adds to $CH_2$ to give the 3 degree carbocation, then $Br^-$ adds there, giving tert-butyl bromide.
Final Answer:
Only an unsymmetrical alkene follows the rule, so 2-methylpropene is the answer.
\[ \boxed{D} \]