The bohr model of the atom addressed the problem of
a.) why electrons didn't fall into the positive nucleus
b.) how so many positive charges could be crowded into such a small space in the nucleus
c.) how so much energy could be released from an atom
d.) all of the above

The Bohr model of the atom addressed the problem of why electrons didn't fall into the positive nucleus.

In atomic physics, the Rutherford–Bohr model or Bohr model, introduced by Niels Bohr in 1913, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with attraction provided by electrostatic forces.

The correct answer between all the choices given is the first choice or letter A. I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.