Meaning of proton affinity

Proton affinity is the tendency of a molecule to accept a proton, i.e. to act as a Lewis base toward $H^+$.
Greater proton affinity means the species is more basic in the gas phase.

Given species

We compare:

• $H_2S$
• $NH_3$
• $PH_3$
• $H_2O$

All of these contain a lone pair, so each can accept a proton.

Key idea for comparison

Proton affinity depends mainly on how available the lone pair is for donation.

• A more available lone pair $\Rightarrow$ higher basicity $\Rightarrow$ higher proton affinity.
• Smaller, more electronegative atoms hold the lone pair more tightly.
• Very weak overlap / inert pair character in heavier hydrides can also reduce basicity.

Compare the hydrides

(i) $NH_3$ vs $PH_3$

$NH_3$ is much more basic than $PH_3$.

In $PH_3$, the lone pair is in a larger, more diffuse orbital and is less effectively available for bonding; hence $PH_3$ is a much weaker base.

Therefore, $NH_3 > PH_3$ in proton affinity.

(ii) $H_2O$ vs $H_2S$

Between group 16 hydrides, $H_2O$ is more basic than $H_2S$ in terms of proton affinity. Sulfur is larger and the bonding/lone-pair characteristics make $H_2S$ a weaker base than water.

Therefore, $H_2O > H_2S$ in proton affinity.

Now compare $NH_3$ and $H_2O$

Nitrogen is less electronegative than oxygen, so the lone pair on $N$ in $NH_3$ is held less tightly and is more available for protonation.

Hence, $NH_3 > H_2O$ in proton affinity.

Overall order

The approximate order is: $NH_3 > H_2O > H_2S > PH_3$ (or $NH_3$ clearly highest among the given options).

Correct option

Therefore, the substance with the highest proton affinity is: $\boxed{NH_3}$

So, Option B is correct.