A mixture is a combination of two or more substances without any chemical combination. Each of the constituent elements retain their individual identity in a mixture. A pure substance, on the other hand, can be made up of one or more elements chemically combined in such a way that individual identity is lost.
Looking at the given options, we can see that option A and C are the mixture, while B and D are not. In case of A, both the reactant and product are mixtures, since individual atoms can be seen and identified. In case of B, a mixture consisting of two different elements converts to a pure substance by combination of these different atoms. In the case of C, a pure substance consisting of two different elements breaks up to release different atoms (all of which can be individually identified) and hence is a mixture. In the case of D, atoms combine to form a pure substance and lose their identity during the process. This is not a mixture.
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