Explanation

(c)
1 mole of $\left( NH_{4} \right)_{3}PO_{4}$ contains 12 moles of hydrogen atoms.
$\therefore 12$ moles of hydrogen atoms $\equiv$ 1 mole of $\left( NH_{4} \right)_{3}PO_{4}$
$\therefore 1$ moles of hydrogen atom $= \frac{1}{12}$ mole of $\left( NH_{4} \right)_{3}PO_{4}$
$\therefore 6.36$ moles of hydrogen atom $= \frac{1}{12} \times 6.36$
$= \frac{6.36}{12}$ mole of $\left( NH_{4} \right)_{3}PO_{4}$
1 mole of $\left( NH_{4} \right)_{3}PO_{4}$ $= 4$ moles of oxygen
So, $\frac{6.36}{12}$ mole of $\left( NH_{4} \right)_{3}PO_{4} = \frac{4 \times 6.36}{12} = 2.12$ mol