Answer: The correct answer is Choice B, 12.3 years.
A z-score of about -0.523 would corresponding to the area which divides the normal distribution at 70% and 30%.
Now, we just need to write and solve an equation for that z-score.
(x - 15) / 5.2 = -0.523
x - 15 = -2.7196
x = 12.283
We can round it to 12.3 years old.