In this task, you need to provide the correct option for a given problem from the provided options.

Problem: 7 machines at a certain factory operate at the same constant rate . if 6 of these machines , operating simultaneously , take 42 hours to fill a certain production order , how many fewer hours does it take all 7 machines , operating simultaneously , to fill the same production order ?
Options: a ) 3 , b ) 4 , c ) 5 , d ) 6 , e ) 7
d