Comparision and Parrallelism goes hand in hand, always keep an hawk eye for the Parallelism and Comparison Indicators, as soon as you see it find the two item which are being talked about and make sure that they are parallel.
E.g:
I love doing X and Y. here and is a parralelism indicator and thus both X and Y should be parallel to each other.
I love doing X more than doing Y. Here than is a comparision indicator, therefore first find out which two elements are being compared and then make sure that they are parallel. here the action of doing one thing is getting compared to action of doing another thing. therefore the action should be parallel to the other action
That being said:
Most citizens of nineteenth-century London believed that remaining respectable was more important and more difficult than to question the virtues of the age.
So what is being compared here?
Remaining X is more important than _________ so the no-underlined part is a gerund therefore we need a gerund to make it parallel. therefore answer will be B
I hope this helps. I am open for suggestions and questions