Which of the answer options is a collective noun for a group of hippopotamuses?

In English, the plural of hippopotamus is "hippopotamuses" or "hippopotami". "Hippos" is also acceptable as a shortened plural. Hippos are gregarious, living in groups of up to thirty animals. A group is called a pod, herd, dale, or bloat.

The Latin name “hippopotamus” derives from the Greek “hippos” meaning ‘horse’ and “potamos” meaning ‘river’ i.e. "horse of the river". It is the third largest land animal after the elephant and rhinoceros and their closest living relatives are whales, dolphins and porpoises from which they diverged about 55 million years ago.

Despite being semiaquatic and having webbed feet, an adult hippo is not a particularly good swimmer nor can it float. It is rarely found in deep water but when it is, the animal moves by porpoise-like leaps from the bottom.

With the exception of eating, most of a hippo's life occurs in the water. Hippos leave the water at dusk and travel inland, sometimes up to 10 km (6 mi), to graze on short grasses, their main source of food. They spend four to five hours grazing and can consume 68 kg (150 lb) of grass each night.