The statement "The classroom was a zoo" is best described as:

The correct option is d.

The statement "The classroom was a zoo" is a metaphor. A metaphor is a figure of speech that directly compares two unlike things without using "like" or "as." In this case, the classroom is directly equated to a zoo to suggest that it was chaotic, noisy, or unruly, similar to how a zoo might be perceived.

• a. Irony involves a contrast between what is said and what is actually meant, or between expectation and reality.
• b. Symbolism is the use of objects or ideas to represent something else.
• c. Hyperbole is an extreme exaggeration used for emphasis or effect. While the statement might feel like an exaggeration, its primary function is the direct comparison.

The final answer is $\boxed{\text{d. Metaphor}}$.