Assertion : All s-block elements are found in nature.

Reason : 4f and 5f series are kept below periodic table.

The correct option is "Assertion and reason, both are true and reason is not correct explanation of assertion."

Analysis of the Assertion:
All s-block elements are found in nature. The s-block elements consist of Group 1 (alkali metals, excluding hydrogen which is a non-metal but sits there, and including francium which is highly radioactive but exists in nature in trace amounts as decay products of uranium and thorium) and Group 2 (alkaline earth metals, including radium which is also radioactive but found in uranium ores). Therefore, all s-block elements (Lithium, Sodium, Potassium, Rubidium, Cesium, Francium, Beryllium, Magnesium, Calcium, Strontium, Barium, and Radium) exist naturally on Earth, even if some like Francium and Radium are present in extremely minuscule trace quantities due to radioactive decay. Thus, the assertion is true.

Analysis of the Reason:
The 4f (lanthanoids) and 5f (actinoids) inner transition series are kept separately at the bottom of the periodic table. This is done to maintain the structure and symmetry of the main periodic table and to prevent it from becoming overly wide. Thus, the reason statement itself is true.

Evaluation of the Explanation:
The position of the 4f and 5f block elements (f-block) at the bottom of the periodic table has no logical connection or physical relevance to the natural occurrence of s-block elements. The two statements describe completely independent concepts within inorganic chemistry. Therefore, while both statements are individually true, the reason is not the correct explanation for the assertion.