The most useful item for Risa to pack for her vacation, given the incoming low pressure system, would be B. a rain coat. Low pressure systems are often associated with cloudy skies, rain, and inclement weather. This means that Risa is likely to experience wet conditions during her trip, making a rain coat crucial to staying dry and comfortable.
Packing a rain coat will allow Risa to enjoy her vacation despite the unfavorable weather conditions. Other items, such as sunglasses, a bathing suit, or shorts, would not be suitable since they are typically associated with sunny and warm weather. In summary, the rain coat is essential for adapting to the expected conditions of a low pressure system, ensuring that Risa can still have an enjoyable and practical trip.