The correct answer is C: The D region absorbs signals at these frequencies during daylight hours. Why long-distance communication on the 40-, 60-, 80-, and 160-meter bands is more difficult during the day is that the D region absorbs signals at these frequencies during daylight hours. D region is ionized by sunlight, creating absorption that affects lower frequencies. For amateur radio operators, this explains why lower bands work better at night. Understanding this helps explain day/night propagation differences.