The correct option is B get along with
The context refers to a new colleague and his relationships with people in the office. The adjective 'difficult' indicates someone awkward or not easy to please. Option B "get along with" means having a friendly relationship with someone. When we combine it with difficult, it indicates someone not easy to have a friendly relationship with. Thus, this is correct here.
"To get on with something" refers to continuing with a task. Here, the reference is to a person and his interpersonal relationships. Thus, option A is incorrect.
"To get over" something or somebody is to return to one's usual state of health, happiness, etc. after an illness, a shock or an end of a relationship, etc.
"To get off" is to leave work with permission.
Thus, option B is correct.