I would say between A and B. "Open to" means "willing to consider." In this sense, Jack neither "wants to" nor "does not want to" date Maria. A common expression is "I am open to suggestions," which means something like "I am willing to listen to suggestions." However, there is no implication on whether I will accept or reject the suggestions.

An upvote for Paul Hsieh. Neither A nor B. If is a person is being honest, "open to" means willing to consider a suggestion. In this case, Jack doesn't want "to date" Maria and he doesn't want "not to date" her. It a normal context, it would mean that he had not considered her romantically, a friend mentioned that Maria liked him, and he responded with "hmmm.... I'm open to dating her." His meaning is that he has no real interest, but she has not made a bad impression and he's willing to spend some time with her to see if she makes a good impression. If a person is being diplomatic (politely dishonest), "open to" is actually "no." Jack knows Maria. Jack has not shown any interest in dating Maria because he either dislikes her or simply has no romantic interest. Some friend says that she likes him and instead of saying that something negative, he gives a diplomatic neutral non-committal response.

Means he is not opposed to the idea. From your choices, probably A is most appropriate.