Q.Assertion: Pure iron, when heated in dry air, is converted to a layer of rust. Reason: Rust has the composition $\text{Fe}_3\text{O}_4$.

A large negative standard reduction potential signals a strong tendency to lose electrons (be oxidised), which is the defining behaviour of a strong reducing agent, not an oxidising agent — this makes statement (d) the incorrect one among the four options.

Checking each statement:

1. $Co^{3+}$ ($d^6$) and $Fe^{2+}$ ($d^6$) — both have the same $d^6$ electronic configuration, so under comparable ligand-field conditions they have the same number of unpaired electrons. This statement is correct.
2. Silver is indeed the element with the highest electrical conductivity of all known metals at room temperature (even higher than copper) — a well-established physical fact. This statement is correct.
3. The Hume-Rothery size-factor rule for substitutional solid-solution alloys states that if the atomic radii of the solute and solvent metals differ by more than about 15%, solid solubility is very limited; a difference of less than $15\%$ favours extensive substitutional alloy formation. This statement is correct. …

The electrochemical series is arranged by increasing standard reduction potential; metals at the bottom (most positive $E°$, the noble metals) are the weakest reducing agents because they least readily give up electrons.

Metals at the top of the electrochemical series (very negative $E°$, e.g. Li, K, Ca, Na) lose electrons most readily and are the strongest reducing agents. Moving down the series, the standard reduction potential becomes progressively more positive, meaning the metal has a decreasing tendency to be oxidised (to lose electrons) and an increasing tendency for its ion to be reduced. Metals near the bottom, such as Cu, Ag and Au ('noble' metals), therefore act as weak reducing agents themselves — it is the …