Explanation:
A mixture is when two or more substances combine physically together. However, in water, two hydrogen atoms combine with one oxygen atom chemically, forming a new substance that has properties different from hydrogen alone or oxygen alone. Therefore, water is not a mixture; it is a compound and it is pure substance.
whereas Air is mixture of gases, Sea water is a mixture many different substances, Soil is composed of small pieces of a variety of materials, so it is a heterogeneous mixture
Hence option C is correct