Simplify each option or check cross-products against the reference ratio 1:2.<br><br>
&bull; 2:4 ÷ 2 = 1:2 → equivalent ✓<br>
&bull; 3:6 ÷ 3 = 1:2 → equivalent ✓<br>
&bull; 4:8 ÷ 4 = 1:2 → equivalent ✓<br>
&bull; 15:28: 15 × 2 = 30 &ne; 28 → NOT equivalent &cross;<br><br>
Final Answer: 15:28