Q.Among the transition metals of 3d series, the one that has highest negative M²⁺/M standard electrode potential is

Standard electrode potential (E⁰) is the standard emf of a cell in which molecular hydrogen at standard pressure/temperature is oxidised to solvated protons at the standard hydrogen electrode -- the universal zero reference. A large negative E⁰ marks a powerful reducing agent (the metal loses electrons readily).

Across the 3d series' M²⁺/M couple, values trend from strongly negative at Ti (-1.63 V) toward less negative values, with copper alone showing a positive value (+0.34 V) -- elemental copper is thermodynamically more stable than Cu²⁺, which is why copper resists dissolving in dilute non-oxidising acids. Mn and Zn both deviate, showing more-negative-than-trend values, because of the extra stability of Mn²⁺'s half-filled d⁵ and Zn²⁺'s fully-filled d¹⁰ configurations (making those ions harder to reduce further).

The M³⁺/M²⁺ couple gives the relative stability of a metal's +3 versus +2 state directly. NEGATIVE values for Ti (-0.37 V), V (-0.26 V) and Cr (-0.41 V) mean the higher (+3) state is preferred for these metals -- reducing a stable ion like Cr³⁺ needs a strong reducing agent such as zinc (E° = -0.76 V). POSITIVE values for Mn (+1.51 V), Fe (+0.77 V) and Co (+1.81 V) mean the lower (+2) state is preferred. Manganese's especially high +1.51 V shows Mn²⁺ (3d⁵, half-filled) is strongly favoured over Mn³⁺ (3d⁴); iron's comparatively low +0.77 V means both Fe³⁺ and Fe²⁺ coexist readily under normal conditions, since neither the 3d⁵ (Fe³⁺) nor the 3d⁶ (Fe²⁺) configuration carries a decisive extra-stability edge the way Mn²⁺ does. This same reasoning explains why Cr²⁺ (readily oxidised toward the more-stable Cr³⁺, so a strong reducing agent) and Mn³⁺ (readily reduced toward the more-stable, half-filled Mn²⁺, so a strong oxidising agent) show opposite redox character, and why Fe³⁺ is generally a somewhat stronger reducing/oxidising-resistant pairing than Cr²⁺ given their respective E⁰ magnitudes.