Kimani stops at a watering hole, where hippos grunt in mud-baths,

Baboons play in a nearby tree, like children out at camp

The underlined portion of the stanza is an example of which sound device?

onomatopoeia

assonance

alliteration

rhyme

If the underlined portion of the stanza is where hippos grunt, then it is an example of the sound device known as onomatopoeia.

Onomatopoeia is a figure of speech in which sounds coming from nature are imitated. So, the verb grunt evokes some animal sounds that can be heard in nature.