Simplify the ratio 15 : 12 by dividing both terms by their highest common factor, which is 3.<br><br>
&bull; 15 ÷ 3 = 5, 12 ÷ 3 = 4<br>
&bull; 15 : 12 = 5 : 4<br><br>
Final Answer: A: 5 : 4