If the chemical elements are arranged according to increasing atomic weight, those with similar physical and chemical properties occur after each interval of seven elements. Ca,Sr do follow octave rule.

Explanation:

The Octet Rule states that atoms tend to gain, lose, or share electrons in order to achieve a full outer shell of eight electrons. This rule applies to elements in the s and p blocks of the periodic table.

In the given options, we need to identify the pair of elements that do not follow the Octet Rule. Let's analyze each option:

a) Na, K:
Sodium (Na) has one valence electron and potassium (K) has one valence electron as well. Both elements can easily lose this electron to achieve a stable electron configuration. Therefore, this pair follows the Octet Rule.

b) Ca, Sr:
Calcium (Ca) has two valence electrons and strontium (Sr) has two valence electrons. Both elements can easily lose these electrons to achieve a stable electron configuration. Therefore, this pair also follows the Octet Rule.

c) F, Cl:
Fluorine (F) has seven valence electrons and chlorine (Cl) has seven valence electrons as well. Both elements tend to gain one electron to achieve a stable electron configuration. Therefore, this pair follows the Octet Rule.

d) O, S:
Oxygen (O) has six valence electrons and sulfur (S) has six valence electrons as well. Both elements tend to gain two electrons to achieve a stable electron configuration. Therefore, this pair also follows the Octet Rule.

From the analysis above, we can see that all the given pairs of elements follow the Octet Rule except for the pair of elements Ca and Sr. Therefore, the correct answer is option B.

Summary:
The pair of elements that do not follow the Octet Rule is Ca and Sr. All other pairs of elements in the given options follow the Octet Rule.

This rule is valid upto Ca. After calcium there is d block present due to which there is difference of 18 elements and not 8.