Given the task definition and input, reply with output. In this task, you need to provide the correct option for a given problem on probability from the provided options.

Problem: a box contains 3 red balls and 4 black balls . one by one , every ball is selected at random without replacement . what is the probability that the fourth ball selected is black ? 
Options: a ) 7 / 4 , b ) 1 / 2 , c ) 1 / 2 , d ) 5 / 8 , e ) 2 / 3

a