Remark
Rule: The biggest angle is opposite the longest line. The smallest angle is opposite the shortest line. The middle angle is opposite the middle sized angle. Put another way.
Solution
Since the sides go in order of
15 > 12 > 8
c > b > a
<C > <B > A
A: <A should be the smallest, not the largest
B: This is the answer. It is in the correct order.
C: Wrong <B should be larger than <A
D: Wrong <B cannot be the largest.
