Depending on how you did (b), I think you just need to conduct a marginal analysis since it asks you to find the point to cease production.
In (b):
MC = marginal cost only (as the farmer owns the lake, therefore bears the externalities)
MR = marginal social benefit
So now consider (c):
MC = marginal cost+marginal external cost (as the company owns the lake, therefore externalities have now added to the cost)
MR = marginal social benefit
You'd still have to check with your tutor to be certain, because the question doesn't not really specify any of the assumptions made.