Number of fatalities means number of deaths among the miners.
Enforcing the Federal Coal Mine Health and Safety Act would definitely decrease the number of fatalities.
Therefore, the time period when the coal mine enforced the Federal Coal Mine Health and Safety Act to help miner would be the time period with the least number of fatalities.
Observing the given table and based on the above explanation, the best choice would be:
d. between 1975 and 1985