1. **State the problem:** We have a function $f(x) = 155 + 125x$ representing the total cost to rent a car, where $x$ is the number of days rented and $f(x)$ is the total charge. 2. **Identify the y-intercept:** The y-intercept of the graph $y = f(x)$ is the value of $f(x)$ when $x=0$. 3. **Calculate the y-intercept:** Substitute $x=0$ into the function: $$f(0) = 155 + 125 \times 0 = 155$$ 4. **Interpret the y-intercept:** The y-intercept represents the cost when no days are rented, which corresponds to the flat insurance fee. 5. **Conclusion:** The flat insurance fee is $155$, so the best interpretation is: **D The car’s flat insurance fee is 155.**