The correct statement is option 'D': Dalton was the first to state that the smallest indivisible portion of matter is an atom.

John Dalton, an English chemist and physicist, is considered the father of modern atomic theory. He proposed the atomic theory in the early 19th century, which revolutionized our understanding of matter. Here is a detailed explanation of Dalton's atomic theory and why it is the correct statement:

Dalton's Atomic Theory:
1. Elements consist of tiny particles called atoms: Dalton stated that all matter is made up of extremely small particles called atoms, which are indivisible and indestructible. Atoms of the same element are identical in mass and properties, while atoms of different elements have different masses and properties.

2. Atoms combine in fixed ratios to form compounds: Dalton proposed that atoms combine with each other in simple, whole number ratios to form compounds. For example, in water (H2O), Dalton suggested that two atoms of hydrogen combine with one atom of oxygen.

3. Chemical reactions involve the rearrangement of atoms: According to Dalton, chemical reactions occur when atoms are rearranged to form new compounds. However, atoms themselves are not created or destroyed during these reactions.

4. Compounds have constant composition: Dalton's theory emphasized that compounds always contain the same elements in the same proportions by mass. This principle is known as the law of constant proportions or the law of definite proportions.

5. Atoms are indivisible and indestructible: Dalton believed that atoms are the smallest particles of matter and cannot be further divided. He considered them to be indestructible, meaning that chemical reactions only involve the rearrangement of atoms and not their destruction.

Significance of Dalton's Atomic Theory:
Dalton's atomic theory laid the foundation for modern chemistry and revolutionized our understanding of the nature of matter. It provided a framework for explaining various chemical phenomena, such as the behavior of gases, the formation of compounds, and the conservation of mass in chemical reactions. Dalton's theory also paved the way for further advancements in the field of atomic structure, leading to the discovery of subatomic particles like protons, neutrons, and electrons.

In conclusion, Dalton was the first to propose the concept of atoms as the smallest indivisible particles of matter. His atomic theory, which includes ideas like the conservation of mass and the fixed ratios of atoms in compounds, is considered one of the fundamental principles of modern chemistry.