Which of the following elements forms a covalent electron-deficient hydride?

The nucleus of tritium contains

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: Loss of electron from hydrogen atom results in a nucleus of $\sim 1.5 \times 10^{-3} \ pm$ size.
Reason $(R)$: Proton $(H^+)$ always exists in combined form.
In the light of the above statements,choose the most appropriate answer from the options given below:

Match List-$I$ with List-$II$

List-$I$ Type of Hydride	List-$II$ Example
$A$. Electron deficient hydride	$I$. $MgH_2$
$B$. Electron rich hydride	$II$. $HF$
$C$. Electron precise hydride	$III$. $B_2H_6$
$D$. Saline hydride	$IV$. $CH_4$

Choose the correct answer from the options given below:

The sum of the number of neutrons and protons in the radioisotope of hydrogen is

Among the following,which has the lowest enthalpy of fusion?