What is the obverse of the proposition "Some chemicals are not toxic substances"?

A) Some chemicals are nontoxic substances.
B) Some toxic substances are non-chemicals.
C) No toxic substance is a non-chemical.
D) No chemical is a nontoxic substance.
E) All nontoxic substances are chemicals.

The obverse of a proposition involves changing the quality (affirmative to negative, or vice versa) and negating the predicate. The original proposition "Some chemicals are not toxic substances" is particular negative. Its obverse changes it to a particular affirmative statement while negating the predicate, resulting in "Some chemicals are nontoxic substances."