Which of the following is least soluble?

The hydroxide of which of the following metals reacts with both acid and alkali?

The correct order of basic strength of oxides of alkaline earth metals is:

What is the role of gypsum,$CaSO_{4} \cdot 2H_{2}O$,in the setting of cement? Identify the correct option from the following:

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ Beryllium has a less negative value of reduction potential compared to the other alkaline earth metals.
Reason $R:$ Beryllium has large hydration energy due to the small size of $Be^{2+}$ but a relatively large value of atomization enthalpy.
In the light of the above statements,choose the most appropriate answer from the options given below.

The use of Plaster of Paris is ..........