Which of the following is the most acidic?

If an electron is transferred from $B$ to $A$,then the reaction is possible when:

How many elements in the periodic table are gases? Provide their positions in the periodic table.

Which of the following oxides is most amphoteric?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R.$
Assertion $A :$ Metallic character decreases and non-metallic character increases on moving from left to right in a period.
Reason $R :$ It is due to increase in ionisation enthalpy and decrease in electron gain enthalpy,when one moves from left to right in a period.
In the light of the above statements,choose the most appropriate answer from the options given below :

Similarity in chemical properties of the atoms of elements in a group of the Periodic table is most closely related to