In Aldous Huxley's Brave New World, infants develop a fear of books after books are repeatedly presented with a loud noise. In this fictional example, the loud noise is a (n)

unconditioned stimulus

Explanation:

Unconditioned stimulus -

It is the stimulus, which is naturally, unconditionally and automatically triggers a response is referred to as unconditioned stimulus.

This stimulus takes place without any prior thought or knowledge.

For example,

The smell of the food, automatically makes the person hungry.

Hence, from the given scenario of the question,

The loud noise is an unconditioned stimulus.