A useful way to check facts about a metalloprotein is to compare it against the other classic metalloprotein families, since most wrong statements in this type of question are true facts, just borrowed from a different protein class.
- "It is an electron transfer protein": plastocyanin belongs to the same functional category as cytochromes and ferredoxins, proteins whose only job is to move a single electron along a chain, here specifically between cytochrome $b_6f$ and photosystem I in photosynthesis. It does not bind or activate a substrate the way an enzyme does. This statement is correct.
- "It is intensely colored in its oxidised form": this is the defining feature of the "blue copper" (type 1 copper) protein family that plastocyanin belongs to, named for exactly this property; the intense blue comes from a cysteine-sulfur to $Cu(II)$ charge transfer band that is only present when the copper is oxidised, and vanishes on reduction to colorless $Cu(I)$. Correct.
- "Its central metal ion has a distorted tetrahedral geometry": type 1 copper sites, unlike the square planar geometry typical of most $Cu(II)$ complexes, are held in a distorted (compressed) tetrahedral geometry by the rigid protein scaffold, a geometry that is actually a compromise favoring fast electron transfer over ideal bonding for either oxidation state. Correct.
- "It contains $S^{2-}$ bridged metal cluster": bridging sulfide ions connecting two or more metal centers is the hallmark of iron-sulfur proteins, like $2Fe-2S$ or $4Fe-4S$ ferredoxins, a completely different protein family from blue copper proteins. Plastocyanin has exactly one metal (copper) and no sulfide bridge; its sulfur atoms come only from cysteine and methionine side chains binding that single copper. This statement describes the wrong protein family and is incorrect for plastocyanin.
Sorting the four statements this way confirms that three of them (electron transfer role, color tied to oxidation state, distorted tetrahedral copper site) are genuine, well known features of plastocyanin, while the fourth borrows a fact that belongs to iron-sulfur cluster proteins instead.
Let's summarize:
- Plastocyanin = single copper, blue in the oxidised $Cu(II)$ state, distorted tetrahedral site, pure electron shuttle.
- Sulfide-bridged clusters are a different protein family (iron-sulfur proteins), not plastocyanin.
The correct options are (A), (B), and (C).