1. Find the oxidation state of nitrogen in each compound

We use the standard rules:

• Hydrogen usually has oxidation state $+1$
• Oxygen usually has oxidation state $-2$
• Elements in free state have oxidation state $0$
• Sum of oxidation states equals the charge on the species

2. Calculate for each substance

(i) $HNO_3$

Let oxidation state of $N$ be $x$.

$(+1) + x + 3(-2) = 0$ $1 + x - 6 = 0$ $x = +5$

So, in $HNO_3$, nitrogen is $+5$.

(ii) $NO$

Let oxidation state of $N$ be $x$.

$x + (-2) = 0$ $x = +2$

So, in $NO$, nitrogen is $+2$.

(iii) $NH_4Cl$

This is better viewed as $NH_4^+Cl^-$. Find oxidation state of $N$ in $NH_4^+$. Let it be $x$.

$x + 4(+1) = +1$ $x + 4 = 1$ $x = -3$

So, in $NH_4Cl$, nitrogen is $-3$.

(iv) $N_2$

$N_2$ is an element in its free state.

$\text{Oxidation state of } N = 0$

So, in $N_2$, nitrogen is $0$.

3. Arrange in decreasing order of oxidation state

The oxidation states are:

$HNO_3 : +5, \quad NO : +2, \quad N_2 : 0, \quad NH_4Cl : -3$

Decreasing order means highest to lowest:

$HNO_3 > NO > N_2 > NH_4Cl$

4. Match with the options

• A: $HNO_3, NO, NH_4Cl, N_2$ ❌ because $-3$ should come after $0$
• B: $HNO_3, NO, N_2, NH_4Cl$ ✅
• C: $HNO_3, NH_4Cl, NO, N_2$ ❌ incorrect order
• D: $NO, HNO_3, NH_4Cl, N_2$ ❌ incorrect order

5. Final answer

The correct option is B.