An element $\mathrm{X}$ from group $2$ of the periodic table reacts with an element $\mathrm{Y}$ from group $17$ to form a compound.

What is the valency of $\mathrm{X}$ element?

The following diagram shows a part of the periodic table containing first three periods in which five elements have been represented by the letters $\mathrm{a}, \mathrm{b}, \mathrm{c}, \mathrm{d}$ and $\mathrm{e}$ (which are not their chemical symbols):

Select the letter which represents an alkali metal. 

The following diagram shows a part of the periodic table containing first three periods in which five elements have been represented by the letters $\mathrm{a}, \mathrm{b}, \mathrm{c}, \mathrm{d}$ and $\mathrm{e}$ (which are not their chemical symbols):

Select the letter which represents a noble gas. 

The following diagram shows a part of the periodic table containing first three periods in which five elements have been represented by the letters $\mathrm{a}, \mathrm{b}, \mathrm{c}, \mathrm{d}$ and $\mathrm{e}$ (which are not their chemical symbols):

Select the letter which represents a halogen.