Which of the following phenomena may be classified as having heavy intensity?

In meteorological terms, "heavy intensity" typically refers to phenomena that significantly affect visibility and can pose hazards to aviation and safety. A sandstorm, or dust storm, can result in drastically reduced visibility due to the large amounts of dust in the atmosphere, making it difficult to see more than a few meters ahead. This can severely impact flight operations, leading to the classification of a sandstorm as a phenomenon with heavy intensity.

Other weather phenomena such as hail, thunderstorms, and funnel clouds, while potentially dangerous, do not typically have the same visibility-impairing characteristics as sandstorms. Hail can cause damage but is not generally described in terms of visibility. Thunderstorms involve various types of precipitation and can produce strong winds, but they don't always result in heavy visibility issues relative to how sandstorms do. Funnel clouds, while indicative of severe weather, are specific formations that don't themselves represent a broad visibility impairment like a sandstorm can. Thus, a sandstorm stands out as the phenomenon classified under heavy intensity due to its significant impact on visibility and safety.