Choice A is the correct answer. Simplifying inequalities to equivalent forms is a key algebraic skill.

1. Subtract 6: $3x + 6 - 6 < 15 - 6$, giving $3x < 9$
2. Divide by 3: $\frac{3x}{3} < \frac{9}{3}$, giving $x < 3$
3. Equivalence: $x < 3$ means exactly the same thing as the original inequality

?�� Strategic Tip: Equivalent inequalities have identical solution sets even though they look different.

Choice B is incorrect because it reverses the inequality direction without justification. Choice C is incorrect because it might come from dividing 21 by 3, where 21 comes from adding 6 and 15. Choice D is incorrect because it multiplies 3 and 6 plus 15 incorrectly, or just takes 15 + 6 = 21.