Option D is the correct answer.
B = Hitting a boundary
GL = good length delivery
SP = short pitched delivery
OP = over pitched delivery
from the given question
P(GL) = 0.4 , P(SP) = 0.4 , P(OP) = 0.2
P(B|GL) = 0.4 , P(B|SP) = 0.8 , P(B|OP) = 0.8
P(SP,B) = P(B|SP)*P(SP) which is 0.4*0.8 = 0.32
To find P(SP|B) we can easily find this using Bayes Therorem we just need to find out the marginalized probability of hitting a boundary
P(B) = P(B|SP)*P(SP) + P(B|OP)*P(OP) + P(B|GL)*P(GL)
P(B) = 0.4*0.80 + 0.2*0.8 +0.4*0.4 = 0.64
P(SP|B) = P(SP,B) / P(B) = 0.32 / 0.64 = 1 / 2 , therefore OPTION D