The correct answer choice is d): the statement that atoms are like sold little balls best reflects Dalton's thinking at the time.
Dalton proposed that atoms are indivisible and indestructible building blocks of matter. So answers a) and b) are incorrect; the understanding that atoms are in fact composed of other particles came later. Dalton recognized that different chemical elements consist of different kinds of atoms, so c) is also incorrect. (Now, we also know that not all atoms of the same chemical element have to be exactly the same, because the number of the neutrons in the nuclei can vary, creating different isotopes of the same element. However, this does not affect the chemical properties of the element, which depend mainly on the number and configuration of electrons in the atom.)
Dalton also realized, by experimentation, that atoms of different elements can combine in different ways to create different materials (chemical compounds).
Dalton's theory, while somewhat incorrect and incomplete, still serves as a foundation of modern chemistry.