Final answer:
d) f(x)=30x−80. The correct profit function is f(x)=30x-80.
Step-by-step explanation:
The correct profit function is f(x)=30x-80. The combined profits of $40 million after 4 years indicates that the profit is increasing over time. The slope of the profit function is positive, indicating that the profit is increasing as the number of years (x) increases.