Final answer:
To convert the average temperature from Celsius to Fahrenheit, the correct formula is (C × 9/5) + 32. This accounts for the scale difference and the freezing point's offset between the two temperature scales.
Step-by-step explanation:
To convert the average temperature in Helsinki, Finland from degrees Celsius (C) to degrees Fahrenheit (F), the correct expression to use is option C) (C × 9/5) + 32. This is because the relationship between the Celsius scale and the Fahrenheit scale is defined by this formula: degrees Fahrenheit (F) = × (degrees Celsius (C) × 9/5) + 32.
When converting temperatures, it's important to remember that while the freezing point of water is 0°C, it is 32°F on the Fahrenheit scale. Therefore, after multiplying the Celsius temperature by the ratio of 9/5 (or 1.8), which accounts for the difference in scale, you must also add 32 to adjust for the freezing point's offset.