Answer:
The correct answer is "…as the war continued and news of the persecution and mass murder of the Jews reached the United Kingdom, it became apparent this was becoming increasingly unlikely."
Step-by-step explanation:
The passage is divided in two paragraphs; the first one is about child refugees and how used to live around 1940; the second one refers to the R. C. M. refers to the Radio Counter-Measures a secretive radio intelligence unit that provided counter-messages against German army.
Since the question is asking about reunification of refugee children with their families the first option is correct: it refers to something that was becoming unlikely; the subject that starts that clause is it in substitution of the main aim of R.C.M. which was this reunion, so A is the correct answer.