Answer: Option (a) is the correct answer.
Explanation:
A substance that cannot be broken down into simpler substances is known as an element.
For example, a piece of sodium metal contains only atoms of sodium.
Whereas a compound is defined as a substance that contains two different elements which are chemically combined together in a fixed ratio by mass.
For example, LiCl is a compound in which atoms are present in 1:1 ratio.
A compound is able to dissociate into its constituent particles but an element cannot.
Both an element and a compound are pure substances.
A mixture is defined as a substance in which two or more number of different substances are mixed together. Particles of a mixture never combine chemically to each other.
So, when black powder on coming in contact with water dissociates as red particles that float to the top, black particles that sink to the bottom, and a green liquid. Then it means a chemical reaction has taken place.
Hence, we can conclude that the black powder is a compound.