Answer:
B
Step-by-step explanation:
AD is an angle bisector and divides the opposite sides into segments that are proportional to the other 2 sides , that is
[tex]\frac{AC}{AB}[/tex] = [tex]\frac{CD}{BD}[/tex] , substitute values
[tex]\frac{12}{21}[/tex] = [tex]\frac{4}{BD}[/tex] ( cross- multiply )
12 BD = 84 ( divide both sides by 12 )
BD = 7 → B
