A standard hydrogen electrode has zero electrode potential because

The correct answer is Option (2) → this electrode potential is assumed to be zero.

• Only the difference between two electrode potentials (the emf of a cell) can be measured, so one electrode is taken as the reference and its potential is fixed at zero by convention. That reference is the standard hydrogen electrode (SHE): platinum in a solution of 1 M \( \ce{H^+} \) with \( \ce{H2} \) gas at 1 bar.

• Every other standard electrode potential is the emf of a cell made with the SHE. For example, a cell of the SHE and a \( \ce{Cu^2+/Cu} \) electrode has an emf of 0.34 V with copper as the cathode, so \( E^{\circ}_{\ce{Cu^2+/Cu}}=+0.34\,\mathrm{V} \). The zero of the SHE is an agreed starting point, not a property of hydrogen.

Why the other options are incorrect

Option (1) hydrogen is easier to oxidize. — Incorrect. How easily a species is oxidised is measured against the SHE; it does not make the SHE potential zero. Many metals, such as zinc and magnesium, are oxidised more easily than hydrogen.

Option (3) hydrogen atom has only one electron. — Incorrect. The number of electrons in the atom has no link to the value given to the electrode; the zero is a choice.

Option (4) hydrogen is the lightest element. — Incorrect. Atomic mass does not decide an electrode potential; the value is assigned, not measured.

NCERT (2026-27 edition):
2.2.1 Measurement of Electrode Potential – “The potential of individual half-cell cannot be measured. We can measure only the difference between the two half-cell potentials that gives the emf of the cell. If we arbitrarily choose the potential of one electrode (half-cell) then that of the other can be determined with respect to this. According to convention, a half-cell called standard hydrogen electrode … represented by Pt(s)|H₂(g)|H⁺(aq), is assigned a zero potential at all temperatures corresponding to the reaction

    \( \displaystyle \ce{H^+(aq) + e^-} \rightarrow \frac{1}{2}\ce{H2(g)} \)”