Water is the purest substance in which hydrogen and oxygen combine with each other in a definite ratio, that is 1 : 8.

An element of a pure substance contains only one kind of atom. Molecule is a group of two or more atoms that are chemically bonded together, i.e. tightly held together by attractive forces. Atoms of the same element or different elements can join to form molecules. The same type of atoms constitutes the molecules of an element. A molecule of ozone (O₃) consists of three atoms of oxygen and thus, it is a molecule of an element. Sugar (C₁₂H₂₂O₁₁), sodium chloride (NaCl) and salt (NaCl) consist of atoms of different elements joined together in definite proportions. Thus, they are molecules of compounds.

Helium is an element. Other options are compounds.

A mixture that does not have a uniform composition throughout its volume is called a heterogeneous mixture. For example: a mixture of salt and sulphur. 

Compound is a pure substance made up of two or more elements chemically combined in a fixed proportion by mass. Compounds cannot be separated into its constituents by simple physical method such as evaporation, filtration, distillation, solvent extraction or by using a magnet. Sugar (C₁₂H₂₂O₁₁) is a compound containing three elements carbon, hydrogen and oxygen in a fixed ratio by mass. Thus, it cannot be easily separated whereas ink, (dye in water), vinegar (acetic acid in water), seawater (water containing soluble salts) are mixtures and they can be easily separated by physical process. For example, dye can be separated from ink by evaporation, vinegar can be separated into its constituents by distillation. Moreover, salts can also be separated from seawater by evaporation.

The solute particles can be separated by filtration.

Substances like common salt, sugar and copper sulphate dissolve in water to form true solutions. The solute particles of a true solution are very small sized, i.e. less than 1 nm in diameter. On the other hand, substances like chalk powder, calcium oxide, etc. are insoluble in water. These substances, when stirred with water, form suspensions. The solute particles in suspension are large, i.e. more than 100 nm in diameter. In between these two categories, there is a third category of substance which dissolves in water or any other liquid to form mixture in which particle size is between 1–100 nm, i.e. between a true solution and a suspension, such mixtures are called colloidal solutions. Due to relatively smaller size, mixtures appear to be homogeneous, i.e. having uniform composition, but actually they are heterogeneous, i.e. particles in them are non–uniformly distributed. Although colloidal particles are small, but they are big enough to scatter light passing through them. As a result, the path of light becomes visible. This scattering of beam of light by colloidal particles is called Tyndall effect. Thus, only option 4 i.e. colloids show Tyndall effect, is correct. 

Homogeneous mixtures have uniform composition throughout mass, i.e. the amount of solute and solvent is the same. Mixture of sugar and water has homogeneity at particle level.

A solution is a homogeneous mixture of one or more solutes dissolved in a particular solvent.

A solution is a homogeneous mixture. It is not a chemical compound.

Pure water is not a mixture because it doesn’t contain impurities.

Only option 4 is incorrect because compound is a combination of two or more elements in fixed composition.

A true solution is a homogeneous mixture of two or more chemically non-reacting substances whose composition can be varied within certain limits. The particles of a true solution are less than 1 nm in diameter. So, they cannot be seen with naked eyes or even under a microscope. True solutions are stable, i.e. the particles do not settle down when kept undisturbed. Moreover, the particles in a true solution are non-reacting, i.e. they will not react to form any new compound and the amount of particles in a true solution is significant. 

Milk of magnesia is a colloid in which the dispersed phase is a solid and the dispersion medium is a liquid. Smoke has a gas as the dispersion medium and a solid as the dispersed phase. Cloud is a colloid in which the dispersed phase is a liquid and the dispersion medium is a gas. Milk is a colloid in which both the dispersed phase and the dispersion medium are liquids.  Thus, option (2) is correct.

Dispersed phase and dispersion medium are the two components of a colloidal solution. The rest of the statements are incorrect as: (1) Combination of all the three phases, i.e. solids, liquids and gases, is not necessary to form a colloid. (2) The solute-like component or the dispersed particles in a colloid form the dispersed phase. (3) A colloid does not settle down when left undisturbed, i.e. a colloid is quite stable.