Identify the correct order of standard enthalpy of formation of sodium halides.

Based on lattice energy and other considerations,which one of the following alkali metal chlorides is expected to have the highest melting point?

Which of the following is not correctly matched?

Which of the following has the lowest thermal stability?

Ions of an element of group $1$ participate in the transmission of nerve signals and transport of sugars and amino acids into cells. This element imparts yellow colour to the flame in flame test and forms an oxide and a peroxide with oxygen. Identify the element and write chemical reaction to show the formation of its peroxide. Why does the element impart colour to the flame?

$X$ $\xrightarrow{N_2, \Delta} Y$ $\xrightarrow{H_2O} Z$ (colourless gas) $\xrightarrow{CuSO_4} T$ (blue colour)
Then,substances $Y$ and $T$ are