Answer: Option (d) is the correct answer.
Step-by-step explanation:
When two different atoms with similar composition are combined together by a chemical reaction then the product formed is known as compound.
For example, Potassium chloride is a compound as it has 1:1 composition, that is, one potassium atom is combined with one chlorine atom.
On the other hand, potassium metal is an element because it consists of only atoms of potassium.
Whereas when two different elements combine together and have different composition then it results in the formation of a mixture.
For example, potassium and chloride is a mixture as we cannot tell its composition without conducting an experiment.
Thus, we can conclude that out of the given options potassium and chlorine is a mixture.