= Solution
\b[Choose process B], the citric-acid/peroxide wet etch. Its GaAs-to-AlGaAs <etch selectivity> is $200/1=200$, giving strong protection of the underlying barrier and a controllable removal rate for the thin cap.
For comparison, A has selectivity one and therefore no useful etch stop. It is also extremely fast: a nominal $20\,\mathrm{nm}$ GaAs layer would be removed in only $0.048\,\mathrm s$. C has selectivity $1000/20=50$, useful but inferior to B, and clears the same cap in about $1.2\,\mathrm s$. D has selectivity four, clears the cap in about $0.060\,\mathrm s$ and additionally risks plasma-induced surface damage or altered interface properties.
B clears the nominal cap in about $6\,\mathrm s$. Its slower underlying etch permits a more forgiving endpoint or modest overetch, although actual rates must be checked for the relevant composition and conditions. Wet-etch lateral undercut and contact-metal compatibility still matter; selectivity alone does not make every mask geometry acceptable.
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