Final answer:
The underlined portion of the sentence 'the black stallion was a bullet' is an example of a metaphor, which directly compares two unlike things without using 'like' or 'as'.
Step-by-step explanation:
The correct term for the underlined portion of the sentence "When the race started, the black stallion was a bullet flying down the track." is metaphor. A metaphor is a figure of speech that directly compares two unlike things without using 'like' or 'as'. Metaphors create a vivid image by suggesting what one thing is similar to another, purely based on their characteristics. In this case, the black stallion is compared to a bullet, implying that the horse is very fast, just like a bullet. This does not involve the literary devices of alliteration (repetition of the same sound at the start of words in close succession), simile (a comparison using 'like' or 'as'), or repetition (recurring words or phrases).