The beginning of the staement "Measles is viewed in developed countries as a mild childhood disease..." is a positive idea. That is it is not a serious ailment. In (a) we have a contrasting word "despite". After this word we would need a negative idea but the fact that there is a vaccine is a positive idea so this does not fit. In (b) we also have a contrasting word "but". So once again we need a negative idea to contrast with the positive beginning and "it can leave children with lifelong disabilities" is indeed a negative idea so this answer is the right one.
I hope that helps you to understand.