Can someone explain this to me please?

Answer:
Option C
Step-by-step explanation:
Since, measure of all sides of the triangle has been given in the picture,
By applying cosine rule in the given triangle,
AC² = AB² + BC² - 2(AB)(BC)cos(B)
11² = 16² + 16² - 2(16)(16)cos(B)
121 = 256 + 256 - 512.cos(B)
121 - 512 = - 512.cos(B)
cos(B) = [tex]\frac{391}{512}[/tex] = 0.763672
B = [tex]cos^{-1}(0.763672)[/tex]
B = 40.2°
Therefore, measure of angle B is 40.2°.
Option C is the answer.