Since BD is the bisector of angle B, the segments to its left are proportional to those to its right. The proportion can be written at least 4 ways, but it is convenient to write it with x in a numerator.
x/(15 cm) = (5.6 cm)/(7 cm)
Multiplying by 15 cm, we get
x = (15 cm)×(5.6/7) = 12 cm
The appropriate choice is
D. 12 cm