Which of the following elements is chemically most reactive?

Given below are two statements: One is labelled as Assertion $A$ and the other is labelled as Reason $R$ :
Assertion $(A)$ : $BeCl_2$ and $MgCl_2$ produce characteristic flame.
Reason $(R)$ : The excitation energy is high in $BeCl_2$ and $MgCl_2$.
In the light of the above statements,choose the correct answer from the option given below :

The number of water molecules in gypsum and plaster of paris respectively are

The ‘milk of magnesia’ used as an antacid is chemically

Portland cement contains :-
$(I)$ $Ca_2SiO_4$ $(II)$ $Ca_3SiO_5$ $(III)$ $Ca_3Al_2O_6$

$s-$block element which cannot be qualitatively confirmed by the flame test is ..... .