The probability of rain on any given day this week is 0.3, independent of other days. What is the probability of at least one rainy day…

Use the complement. The chance of no rain on a single day is $1 - 0.3 = 0.7$. The days are independent, so the chance of no rain on both days is $0.7 \times 0.7 = 0.49$. The chance of at least one rainy day is $1 - 0.49 = 0.51$, choice C. Choice A (0.6) adds the probabilities, $0.3 + 0.3$, which double-counts the case where it rains both days. Choice B (0.09) is the probability of rain on both days, $0.3 \times 0.3$. Choice D (0.3) is the probability for a single day and ignores the second day.