In niels bohr’s model of the atom, electrons move a. like balls rolling down a hill. b. like planets orbiting the sun. c. like popcorn in a popcorn popper. d. like beach balls on water waves.

The correct option is B.
The Niels Bohr's model of atom, which was presented in 1913 depicted atom as a small particle having a positively charged nucleus, that is surrounded by electrons which orbit round the nucleus in a manner that is similar to that observed in the solar system. 

In Niels Bohr's model of the atom, electrons move b. like planets orbiting the sun

Further explanation

According to the Bohr atomic model, electrons circulate around the nucleus on a particular path, so that electrons are not attracted to the nucleus. This path is called the electron shell or energy level, where the path is closest to the nucleus has the lowest energy

Although this atomic model has a disadvantage in describing the trajectory of electrons for atoms that have more electrons (can only predict Hydrogen atoms) but succeeded in explaining the weaknesses of the Rutherford atomic model. The main weakness of the Rutherford atomic model is that Rutherford cannot explain why negatively charged electrons are not attracted to the nucleus of an atom that has positively charged particles

Both of these models (Rutherford and Bohr) are now refined with the atomic theory of quantum mechanics

So the Bohr atomic model can explain the spectrum of hydrogen atoms (energy absorbed/ emitted during electron transitions) whether it is absorption or emission of energy when compared to the Rutherford atomic model

Learn more

Rutherford experiment

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Bohr's model of the atom

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The subatomic particle

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Keywords: Bohr atomic model, Rutherford, quantum mechanics

For the answer to the question above, 
Water will not do much to stop the radiation; therefore, the amount of Uranium should be reduced to decrease the radioactivity.
So my answer would be the last one among the given choices