When a NS (neutral stimulus - bell) is paired with a US (or UCS - unconditioned stimulus - food powder) causing a UR (or UCR - unconditioned response - salivation) over several trials, what does the NS become eventually?

The NS eventually becomes a conditioned stimulus (CS).

Explanation:

In classical conditioning, a conditioned stimulus can be defined as a neutral stimulus that has become associated with an unconditioned stimulus and, eventually, begins to trigger a conditioned response. A classical example is ringing a bell (NS) at the same time you offer food (US) to a dog. The dog will salivate (UR) because of the food, not because of the bell. However, if you repeat this several times, eventually the sound of the bell will go from a neutral stimulus (NS) to a conditioned stimulus (CS). It will begin to trigger the now conditioned response of salivation (CR), even if there is no food.