The figure of speech used in the expression "A big black bug bit a big black dog" is alliteration.

Alliteration is a figure of speech in which the repetition of consonant sounds is used to create a sense of rhythm or emphasis. In this case, the repetition of the "b" sound in the words "big," "black," and "bit" creates a sense of the bug's blackness and its sudden attack on the dog.

The other answer choices are incorrect. Simile is a figure of speech in which two things are compared using the words "like" or "as." Metaphor is a figure of speech in which one thing is compared to another thing without using the words "like" or "as." Oxymoron is a figure of speech in which two contradictory terms are used together. None of these devices are used in the expression "A big black bug bit a big black dog.".

The alliteration in the expression "A big black bug bit a big black dog" is a simple but effective way to create a sense of the bug's blackness and its sudden attack on the dog. It is often used to create a sense of excitement or suspense.