Scientists might predict that all the responses you listed could happen due to solar flares, as solar flares can have various impacts on technology and communication systems. However, if asked to select one of those responses that is commonly highlighted, the prediction that "high-frequency radio waves used for radio communication will be blocked" is often emphasized.
In summary, any of the consequences listed could occur as a result of solar flares, but the blocking of high-frequency radio waves is particularly notable.