Which metal in the first series of transition metals exhibits +1 oxidation state most frequently?

Copper metal is the first series of transition metals, exhibits +1 oxidation state most frequently.

In the first series of transition metals, copper metal (with electronic configuration $\left[\mathrm{Ar}\right]3{\mathrm{d}}^{10}4{\mathrm{s}}^1$) most commonly shows +1 oxidation state because it rapidly loses one electron (present in $4\mathrm{s}$ orbital) to produce stable $3{\mathrm{d}}^{10}$ electronic configuration.

Hence, option C is correct.