The key constraints are two: the target is a protein, and it must be demonstrated within a tissue section (FFPE) without destroying the architecture. Only one assay satisfies both - immunohistochemistry.
IHC applies a primary antibody against the antigen of interest directly onto the deparaffinised slide; after antigen retrieval, a labelled secondary system and a chromogen (commonly DAB) produce a coloured deposit exactly where the protein resides. The pathologist can therefore see which cells and which subcellular compartment express the marker.
FISH is ruled out because it visualises DNA/RNA sequences, not proteins. Western blotting and flow cytometry can both quantify proteins, but each needs the tissue to be disrupted - western blot needs a lysate run on a gel, flow cytometry needs a single-cell suspension - so both forfeit the in-situ localisation the question demands.
\[\boxed{\text{FFPE protein localisation} \Rightarrow \text{Immunohistochemistry (IHC)}}\]