The correct answer is A - III, B - I, C - IV, D - II.

 Key Points

• According to Dalton's theory, matter consists of indivisible atoms, and all atoms of an element have the same mass and similar properties.
  • This explained the law of chemical combination.
  • This theory also stated that compounds are formed when atoms of different elements combine in a fixed ratio.
• According to the law of definite proportions, in a given compound, the elements are always in a constant ratio by weight; for example, in carbon dioxide (CO₂), carbon and oxygen are combined in a fixed ratio of 3:8 by weight.
  • This is also called the law of definite proportions.
• Cations are positively charged ions, meaning they have more protons than electrons, and they are formed when a metal loses its electrons.
• For example, Ca²⁺, K⁺, Ag⁺, Zn²⁺, Na⁺, etc.
• Anions are negatively charged ions, meaning they have more electrons than protons, formed when a non-metal gains electrons.
• For example, OH^-, Cl^-, etc.
• Sulfate is a compound made of sulfur and oxygen, and it is an anion with the formula SO₄^2-, while oxalate is an anion with the formula C₂O₄^2- and ammonium ion (NH₄⁺) is a cation.
• Atomic mass is the mass that is equal to one-twelfth of the mass of a carbon-12 atom, and molecular mass is the sum of the atomic masses of all the elements present in a molecule.
• Therefore, a mole of an element's gram atomic mass and a mole of a compound's gram molecular mass contain the same number of molecules. Thus, option 2 is correct.