Introduction:
Colloids are a type of mixture that consist of two phases: a dispersed phase and a continuous phase. The dispersed phase consists of small particles or droplets that are dispersed throughout the continuous phase. Colloids are different from solutions because the particles in colloids are larger and do not dissolve completely in the continuous phase.

Explanation:

b) Are two-phase systems:
Colloids are two-phase systems because they consist of two distinct phases - the dispersed phase and the continuous phase. The dispersed phase contains the particles or droplets that are dispersed throughout the continuous phase. The continuous phase is the medium in which the dispersed phase is dispersed. Examples of colloids include milk, mayonnaise, and fog. In milk, for example, fat globules are dispersed throughout the water-based continuous phase.

a) Are suspensions of one phase in another:
This statement is incorrect. Colloids are not suspensions of one phase in another. Suspensions are heterogeneous mixtures in which solid particles are dispersed in a liquid or gas. In suspensions, the particles are larger and can settle over time. In colloids, the particles are smaller and remain dispersed throughout the continuous phase due to the repulsive forces between them.

c) Contain only water-soluble particles:
This statement is incorrect. Colloids can contain particles that are soluble or insoluble in the continuous phase. For example, in an oil-in-water emulsion, the oil droplets are insoluble in water but are dispersed throughout it. Similarly, in a gelatin colloid, the gelatin particles are soluble in water but form a gel-like structure.

d) Are true solutions:
This statement is incorrect. Colloids are not true solutions. True solutions are homogeneous mixtures in which the solute particles are completely dissolved in the solvent. In colloids, the dispersed phase consists of particles that are larger and do not dissolve completely in the continuous phase. The particles in colloids can be seen under a microscope and can scatter light, giving colloids their characteristic opaqueness or translucence.

Conclusion:
In summary, colloids are two-phase systems consisting of a dispersed phase and a continuous phase. They are not suspensions of one phase in another, do not necessarily contain only water-soluble particles, and are not true solutions. Colloids have unique properties and can be found in various everyday substances.