Properties of the elements are given below. Where would you locate the following elements in the periodic table?
$(a)$ $A$ soft metal stored under kerosene.
$(b)$ An element with variable (more than one) valency stored under water.
$(c)$ An element which is tetravalent and forms the basis of organic chemistry.

(N/A) Sodium $(Na)$ is located in Group $1$ and Period $3$,or Potassium $(K)$ is located in Group $1$ and Period $4$. These are soft alkali metals that react vigorously with air and moisture,hence stored under kerosene.
$(b)$ Phosphorus $(P)$ is located in Group $15$ and Period $3$. White phosphorus exhibits variable valency and is highly reactive,catching fire in air,so it is stored under water.
$(c)$ Carbon $(C)$ is located in Group $14$ and Period $2$. It is tetravalent (valency of $4$) and its unique ability to form long chains (catenation) makes it the fundamental element of organic chemistry.