Mark knows that animals reflexively blink when air is puffed at their eyes. Now Mark wants to use this fact and classical conditioning to train a rat to blink when it hears a tone. What would the neutral stimulus be in this situation?

the tone is the neutral stimulus

Explanation:

In classical conditioning, as first experimented by Pavlov in his experiment with dogs, the bell that was presented initially was the neutral stimulus which was not capable of eliciting salivation alone. He later paired the neutral stimulus (the bell), with the unconditioned stimulus (meat powder) which naturally elicited salivation. The neutral stimulus then became associated with the unconditioned stimulus to eventually trigger salivation when presented alone.

In the situation of the rat of which Mark wants to train it by applying classical conditioning technique, the tone is the neutral stimulus. Except it is paired with the unconditional stimulus which naturally would make the rat blink, it would only attract the attention of the rat without making it to blink.