Modern warfare has changed from large scale clashes of armies to suppression…

Concept: In a “which statement best sums up the meaning of the passage” question, the correct choice must capture the passage's overall gist — including any tone or judgement the author conveys — not merely restate one sentence from it or introduce a claim the passage never supports.

Application: The passage describes modern warfare shifting toward suppressing civilian populations, and notes that chemical agents — which “do their work silently” — suit that shift. The key signal is the word “regretfully” before the sentence about military figures who view chemical agents as useful tools: this word shows the author disapproves of that view rather than endorsing it. So the passage, taken as a whole, is expressing that resorting to chemical agents in warfare is a troubling, undesirable development — not merely reporting a neutral fact.

Cross-check — why each other statement falls short:

• “Civil strife” names internal social unrest among a population; the passage instead describes a military tactic directed at civilians, not disorder within the civilian population itself — a mismatch with what the passage actually says.

• Restating that chemical agents are seen as useful only repeats the belief some military figures hold; it drops the author's regretful, disapproving framing of that belief, so it is not the passage's own message.

• Calling the use of chemical agents desirable is the opposite of the passage's tone — “regretfully” signals disapproval, not endorsement.

Result: The statement that captures both the description (silent, civilian-targeting agents) and the author's regretful, disapproving stance toward those who see them as useful is the one concluding that the use of chemical agents in warfare would be undesirable — this is what the passage as a whole conveys.