Which of the following has the lowest thermal stability?

Given below are two statements:
Statement $I$ : The $4f$ and $5f$-series of elements are placed separately in the Periodic table to preserve the principle of classification.
Statement $II$ : $S$-block elements can be found in pure form in nature.
In the light of the above statements,choose the most appropriate answer from the options given below :

Valency electrons in alkali metals are

Explain why is $sodium$ less reactive than $potassium$?

What happens if the concentration of $Na^{+}$ ions increases in the body?

Choose the false statement regarding the properties of sodium hydroxide $(NaOH)$.