The following half cell represents, normal hydrogen electrode (NHE) :
Pt|H+ (1M HCl)|H2(g) (1 atm)
The question asks specifically for the normal hydrogen electrode, and the distinction between NHE and SHE is exactly what it is testing. The two are often used loosely as synonyms, but they are defined differently.
The SHE (standard hydrogen electrode) is the rigorous thermodynamic reference. It requires hydrogen ion activity equal to 1, \(a_{H^{+}} = 1\), together with H2 at unit fugacity. Activity is the thermodynamically effective concentration, \(a = \gamma c\), and because interionic attractions make \(\gamma < 1\) in any real solution, unit activity is a hypothetical ideal state rather than something poured from a bottle.
The NHE (normal hydrogen electrode) is the older, practical version defined on a concentration basis — a 1 normal (1 M) acid solution, conventionally 1 M HCl, with hydrogen at 1 atm over a platinised platinum electrode. Being defined by concentration rather than activity, it is realisable in the laboratory, and its potential differs from the SHE by only a few millivolts.
So the option written in terms of \(a = 1\) describes the SHE, not the NHE, and is the correct answer to a different question.
Among the concentration-based options, hydrochloric acid is the right choice because it is a strong monoprotic acid, fully dissociated, so a 1 M solution really does deliver 1 M H+.
1 M H2SO4 fails because it is diprotic: the first ionisation is complete but the second is not, so the hydrogen ion concentration is neither 1 M nor well defined.
1 M H3PO4 fails because it is a weak triprotic acid, only partly dissociated, giving far less than 1 M H+.
The platinised platinum serves as an inert, high-surface-area catalyst for the H+/H2 equilibrium rather than taking part in the reaction.
Hence the NHE is Pt|H+ (1M HCl)|H2(g) (1 atm).
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