Observe the following stoichiometric equation

$$ \mathrm{P}_4+3 \mathrm{OH}^{-}+3 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{PH}_3+3 x^{-} $$

What is the conjugate acid of $x^{-}$?

Identify the correct statements

i. Oxidation of $\mathrm{NaBH}_4$ with $\mathrm{I}_2$ gives $\mathrm{B}_2 \mathrm{H}_6$

ii. $\mathrm{B}_2 \mathrm{H}_6$ burns in oxygen and releases an enormous amount of energy

iii. $\mathrm{B}_2 \mathrm{H}_6$ on hydrolysis gives a tribasic acid

The disproportionation products of ortho phosphorus acid are

Aluminium carbide on reaction with $\mathrm{D}_2 \mathrm{O}$ gives $\mathrm{Al}(O D)_3$ and ' $X^{\prime}$. What is ' $X^{\prime}$ '?