Identify a compound having highest thermal stability.

Which among the following has highest basic strength?

Identify the nitrogen atom of purine base that bonds with $1^{\prime}$ carbon of ribose to form ribonucleoside?

The resistance of a conductivity cell of 0.1 M KCl solution is 120 ohm and conductivity is $1.64 \times 10^{-4} \mathrm{~S} \mathrm{~cm}^{-1}$. What is the value of cell constant?