Answer:
A). An atom is an indivisible hard-sphere.
Explanation:
The 'Atomic Theory' of Dalton is characterized as the earliest model(came in 1803) which described the atoms as the indivisible and resistant spheres. He used the example of watermelon to elaborate that the atoms of a specific element share similar characteristics and the atoms of distinct elements differ in their mass as well as their size. Thus, option A is the correct answer.
