Step 1: Recall the purpose of etching.
Etching a polished steel sample is meant to attack grain boundaries and different phases at different rates so that the microstructure becomes visible under a microscope, so we need a reagent known to work reliably on ferrite and cementite.
Step 2: Match each reagent to its usual metal.
A mixture of ammonia and ferric chloride is the classic etchant for copper alloys like brass and bronze rather than steel, and sodium thiocyanate with ammonium nitrate is not a standard steel etchant at all, so both of those can be set aside.
Step 3: Compare the two remaining candidates.
Sodium picrate solution is indeed used on steels, but it plays a narrower role, selectively darkening cementite while leaving ferrite bright, whereas a dilute mixture of nitric acid in ethyl alcohol, commonly called Nital, is the everyday general purpose etchant that reveals ferrite cementite boundaries and grain boundaries across almost all carbon and low alloy steels, which is why it is the standard laboratory choice.
\[ \boxed{\text{mixture of nitric acid and ethyl alcohol}} \]