What Does the Phrase 'Internalising an External Cost' Mean

Multiple Choice

What does the phrase 'internalising an external cost' mean?

A) Limiting the extent to which domestic firms can outsource production
B) Prohibiting economic activities that create externalities
C) Forcing producers to factor into their production costs the cost of the externalities created in the production of their output
D) Finding a way to address cross-border pollution