Answer:
b) 9
Step-by-step explanation:
Given: Three machines operating independently, simultaneously, and at the same constant rate can fill a certain production order in
hours.
To Find: If one additional machine were used under the same operating conditions, in how many fewer hours of simultaneous operation could the production order be filled.
Solution:
Let the time taken by one machine to fill a certain production alone be
Time taken when three machine operate independently and simultaneously



Let time taken when one additional machine is used 
Time when when one additional machine is used



it takes
when one additional machine is used
Now,
fewer hours taken when one additional machine is used




in
fewer hours of simultaneous operation the production order can be filled if one additional machine is used
Hence option b) is correct.