Final answer:
El Niño typically causes increased rainfall and possible flooding in the western United States, which is the correct answer to the student's question. This occurs because of the shift in weather patterns resulting from changes in sea surface temperatures in the Pacific.
Step-by-step explanation:
El Niño, which is the warm phase of the El Niño Southern Oscillation (ENSO), can have significant impacts on weather patterns around the world. When El Niño occurs, there is a band of warm ocean water that develops in the central and east-central equatorial Pacific. This phenomenon leads to high air pressure in the western Pacific and low air pressure in the eastern Pacific. For the western United States, one of the most common effects of El Niño is increased rainfall and possible flooding. Thus, the correct answer to how weather in the western United States might be affected by El Niño is (a) Increased rainfall and flooding.
Farming and fishing economies in regions impacted by El Niño are particularly vulnerable. In addition to higher precipitation events, El Niño can cause a variety of extreme weather conditions such as fires, drought, and even crop failures in different parts of the world. The impact on the western United States is usually an increase in precipitation due to the shift in weather patterns, which can lead to challenges like flooding and the management of water resources.