Hint: The standard reduction potential of a chemical is the reduction potential of a chemical or molecule under precise standard conditions to lose its electrons or to gain electrons. It is useful in determining the directionality of a chemical reaction.
> We know that the potential of a given chemical can be considered to be negative if the chemical loses its electrons and the potential of a given chemical can be considered to be positive if the chemical accepts its electrons.
> Absolute electrode potential of an electrode cannot be determined without forming a complete cell. Therefore, hydrogen is selected as a standard reference electrode, and describes its electrode potential as zero.
> Coming to given options, option B, Hydrogen is easiest to oxidise, it is not true.
> Coming to option C, Hydrogen has a single electron. Because of a single electron nobody will choose hydrogen as a standard electron.
> Coming to option D, Hydrogen is electronegative. It is also wrong, because electronegativity of hydrogen is very less.
> Coming to option A, It is assumed. Yes we assumed that the standard reduction potential of hydrogen electrode is zero.
So, the correct option is A.
Note: Don’t be confused with the terms reduction potential and oxidation potential.
Reduction potential: It is defined as the tendency of a chemical to gain electrons and it will be reduced.
Oxidation potential: It is defined as the tendency of a chemical to lose electrons and it will be oxidised.