Step 1: Chemical use.
Potassium metabisulphite (K$_2$S$_2$O$_5$) is mainly used in food processing and winemaking as a preservative and antioxidant.
Step 2: Eliminate wrong options.
(A) Weedicide – kills weeds, not true.
(B) Insecticide – controls insects, not applicable here.
(D) Nematode controller – controls soil nematodes, not correct.
Step 3: Conclusion.
Thus, potassium metabisulphite is a preservative.