Final answer:
Each scientist contributed significantly to atomic theory: Dalton theorized that atoms are indivisible and the smallest unit of an element, Thomson discovered the electron, Rutherford identified the nucleus containing protons, and Bohr developed the concept of electron orbital levels.
Step-by-step explanation:
Matching the scientist to his contribution to the atomic theory requires understanding the historical advancements in the structure of the atom. The contributions as per the mentioned scientists are:
- John Dalton is matched with 'b. Atoms are the smallest unit of an element.'
- Niels Bohr correlates with 'd. Electrons are in orbital levels.'
- Ernest Rutherford links to 'c. Protons are at the center of an atom.'
- J. J. Thomson should be associated with 'a. Electrons are scattered within the atom.'
Dalton's Atomic Theory was proposed around 1803 and maintained that elements are composed of atoms, which were considered to be the smallest unit that maintains the identity of that element. J. J. Thomson's work led to the discovery of the electron, and suggested the 'plum pudding' model where electrons are scattered within the atom. Rutherford expanded on this concept by proposing that the atom has a central positive nucleus, while the work of Niels Bohr introduced the idea of electrons residing in orbital levels around the nucleus.