Answer:
B. forcing producers to factor into their production costs , the cost of the externalities created in the production of their output
Explanation:
" internalizing an external cost " -
It is the process of shifting the cost or the burden from negative externality like the pollution , to inside .
And this process is achieved via paying taxes , the government subsidies , tolls etc .
Hence , the correct explanation for " internalizing an external cost " is ( b ) .