1. **Problem statement:** We analyze the temperature conversion function from Celsius ($°C$) to Fahrenheit ($°F$) given by $$f(x) = \frac{9}{5}x + 32$$ where $x$ is the temperature in Celsius and $f(x)$ is the temperature in Fahrenheit. 2. **Understanding proportionality:** A function is directly proportional if it can be written as $f(x) = kx$ for some constant $k$, meaning the graph passes through the origin $(0,0)$ and the ratio $\frac{f(x)}{x}$ is constant. 3. **Check proportionality of $f(x)$:** Here, $f(x) = \frac{9}{5}x + 32$ has a constant term $32$, so it does not pass through the origin. Therefore, it is **not proportional** (neither directly nor indirectly proportional). 4. **Interpretation of temperature changes:** - Increasing Celsius temperature by a factor of 3 does not mean Fahrenheit increases by a factor of 3 because of the added constant 32. - For example, $320\,°F$ does not correspond to half the Celsius temperature of some value because the relationship is linear but shifted. - An increase of $1° C$ corresponds to an increase of $\frac{9}{5} = 1.8° F$, not the same number of degrees. 5. **Summary:** The temperature conversion is a linear function with a positive slope and a positive intercept, so it is **not proportional**. **Final answer:** The correct choice is **"nicht proportional"**.