Introduction:
A mixture is a combination of two or more substances that are physically combined. In a mixture, the individual substances, called constituents, retain their own properties and are not chemically combined.

Explanation:
1. Definition of a mixture:
A mixture is a combination of two or more substances that are not chemically bonded together. The substances in a mixture can be separated by physical means, such as filtration, distillation, or evaporation.

2. Mixture is not a pure substance:
The statement "a) It is a pure substance" is incorrect. A pure substance consists of only one type of particle and cannot be separated into other substances by physical means. In contrast, a mixture consists of different types of particles and can be separated into its constituents.

3. Constituents are not combined chemically:
The correct statement is "b) Its constituents are not combined chemically." In a mixture, the individual substances retain their own chemical properties and are not chemically changed when combined. The constituents of a mixture can be elements or compounds.

4. Constituents retain their individual properties:
The statement "c) Its constituents do not retain their individual properties" is incorrect. In a mixture, each constituent retains its own physical and chemical properties. For example, if you mix salt and sand, you can still see the grains of salt and sand and they retain their individual properties.

5. Homogeneity of a mixture:
The statement "d) It is always homogeneous" is incorrect. A mixture can be either homogeneous or heterogeneous. A homogeneous mixture has a uniform composition and the constituents are evenly distributed. Examples of homogeneous mixtures include saltwater and air. On the other hand, a heterogeneous mixture has a non-uniform composition and the constituents are not evenly distributed. Examples of heterogeneous mixtures include a mixture of oil and water or a mixture of sand and pebbles.

Conclusion:
In conclusion, the correct statement about a mixture is that its constituents are not combined chemically. In a mixture, the individual substances retain their own properties and are not chemically changed when combined. Additionally, a mixture can be either homogeneous or heterogeneous, depending on the uniformity of its composition.

Option'b is mixture of any substance is very soft and warm