Answer :
The statement that best describes the composition of potassium chlorate, KCIO3 is " The proportion by mass of elements combined in potassium chlorate is fixed."
Answer : The correct option is, (1) The proportion by mass of elements combined in potassium chlorate is fixed.
Explanation :
The composition of any compound is studied on the basis of law of fixed proportion. This law stats that in a compound the elements are present in fixed proportion by their mass.
In the given options, the option (1) statement describes the composition of potassium chlorate.
While the other options (2), (3) and (4) statements does not describe the composition of potassium chlorate.
Hence, the correct option is (1).