Concept:The phrasal verb “warn against” is correctly used as is; no substitution is needed.
Explanation:The phrase “warn against” means to advise someone not to do something because it may have negative results.
It is always followed by a gerund (verb + -ing), not an infinitive or a prepositional phrase.
In the sentence, “against committing” follows this rule exactly.
Option a) “to commit” is incorrect because “warn” does not take an infinitive in this construction.
Option b) “for committing” changes the meaning—it implies warning for a past action, not against a future one.
Option c) “against to commit” is grammatically wrong because “against” cannot be followed by “to + verb”.
Thus the original underlined part is correct, and no improvement is needed.
Answer:d) No improvement