Often the need to clean up the atmosphere ---- through major environmental disasters of which the London smog of 1952 still ---- a prime example.
A) was realised / remainsAfter doing my best to understand, I thought it wass time to open this UDS question to the NATIVES. Which one is the right answer? If it is A, what "often" stands for? To show habit in the past, like many other adverbs of frequency? Don't we need a comma just before the prep of "OF"? Could you restate or rephrase this sentence, or divide into two or more sentences so as to make it clearer for a poor non-native?
B) has been realised / remained
C) would be realised / would remain
D) was being realised / will remain
E) had been realised / has remained
Thank you very much in advance.
C, D, and E are all remotely feasible, given the right context; but I think the intended answer is A, in which case the meaning would be:
"On many occasions, people only appreciated the need to clean up the atmosphere after major environmental disasters, of which the London smog of 1952 (still) remains a prime example."
(It would be more natural to use the present perfect for the first verb and the simple present for the second; but that doesn't seem to be an option!)
All the best,
PS: You are right to be a little doubtful about the sentence. Apart from the impersonal use ("it is realized", etc.), the passive forms of "realize" usually only occur when it means "bring about" or "cause to exist". So it has a very strange ring to it.