Which of the following has the highest electrical conductivity in an aqueous solution?

The metal that is extracted from sea water is

When sodium metal is dissolved in liquid ammonia,a blue-coloured solution is formed. The blue colour is due to:

Which of the following metals releases hydrogen gas upon reaction with cold and dilute $HNO_3$?

Which one of the following is an oxide?

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 :