Which of these ideas was part of the earliest model of the atom?
a) atoms can produce streams of charged particles
b) electrons move too fast to describe where they are located
c) neutrons are located in the same place as protons
d) atoms are hard, tiny particles like billiard balls

Option (D) is the correct answer.

Explanation:

Earliest model of the atom was given by Dalton. He stated that atoms are extremely small particles and they cannot be subdivided, created or destroyed.

He also said that for a given element, atoms have identical size, mass and other properties.

Thus, we can conclude that the idea atoms are hard, tiny particles like billiard balls was part of the earliest model of the atom.