The elements A and B obey Newland’s law of the octave. How many elements are there between A and B? 

The Newlands law of octave is stated as if we organized the chemical elements based on increasing atomic weight, elements that have similar physical and chemical properties are seen after the interval of seven elements.

The eighth element to lithium is sodium. Similarly, the eighth element to sodium is potassium.
Thus, if  A is the first element and it goes up to element B such that they are following the Newlands law of the octave. Then there must be 6 elements between them (eighth element) elements A and B.

Hence option (D) is correct.