Quickly spinning the handle of a hand generator, Kristina is able to light three bulbs in a circuit. When she spins the generator slowly, the bulbs are very dim. Which statement best explains why the slow generator is not able to power all three light bulbs?

Voltage is too low

Explanation:

Generators produce a potential difference (voltage) by spinning a coil of wire in a magnetic field by induction.

The produced voltage is proportional to the relative speed of the coil wire with respect to the magnetic field.

Hence, for a given generator, the higher the speed the higher the potential difference.

Therefore, it is concluded immediately that when Kristina spins the generator slowly, less potential difference (voltage) is generated.

Also, you must know that the brightness of the bulbs varies with the current: the higher the current, the brigther the bulbs.

On the other hand, the basic realtion between voltage (V), resistance (R), and current I) is given by Ohm's law: V = R * I. This is, voltage and current are proportional.

Since the generator is producting less voltage, for the same bulbs (which are resistors), the current will be lower too. And, as stated, lower current means lower brightness, which explains why when Kristina spins the generator slowly, the bulbs are very dim.