Which one of the following is the strongest base?

The thermal stability of alkaline earth metal carbonates $MgCO_3$,$CaCO_3$,$BaCO_3$,and $SrCO_3$ decreases as:

Mortar is a mixture of

Among the following,which is water insoluble?

Which of the following orders is correct $:-$

Assertion $(A)$: $MgO$,$CaO$,$SrO$,and $BaO$ are insoluble in water.
Reason $(R)$: In aqueous medium,the basic strength of $MgO$,$CaO$,$SrO$,and $BaO$ increases with an increase in the atomic number of the metal.
The correct option among the following is: