At First sentence
Modern warfare has changed from large scale clashes of armies to suppression of civilian populations"
->talks about shrinking of the level of wars in todays world
In Second Sentence
Chemical agents that do their work silently appear to be suited to such warfare;
->talks about advantages of the chemical agent in wars
In Third Sentence
and regretfully, there exist people in military establishments who think that chemical agents are useful tools for their cause.
->It says There Exist People in military who thinks they are useful
Looking at options
A. As civil Strife would be between To different parties and not armies and population, not the correct answer according to me.(Not Correct)
B. It tells about the 1st and 2nd sentence but Doesn't tell about 3rd sentence so It won't summarize it.(Not Correct)
C. "it basically tells that there are some people who don't desire to send chemical agents to war, yes there has been a word used "regretfully" but the question has mentioned about Likeness of chemical agent from military and not about "undesirability" so it doesn't summarize the question correctly.(Not Correct)
D. It covers all the sentence and summarizes it very well, somepoints may feel like it is not summarizes well but in compared to other options it is the best possible option we have.
(please do correct me if I am wrong anywhere I would love to improve my thinking about this)