Based on the context, to discompose means to cause someone to confuse.
Option (b);
EXPLANATION:
Discompose, according to the dictionary means, to agitate or to make uneasy. It also means to disturb.
In the novel, 'The Tale of two cities' by Charles Dickens, it explain the activities occur in the cities London and Paris.
In the given context, when the author says that 'she looks so discomposed him', he means that she looked at him in such a way that she got confused about him and stopped thinking.