When we describe a situation as "tricky," we mean that it is not straightforward or easy to deal with. It often involves complexities or challenges that require careful handling and consideration. In the given sentence, "We really have to be careful because the situation is rather tricky," the speaker is expressing caution because they recognize that the situation is not simple or easy to navigate.
Therefore, option D, "difficult," is the most suitable choice because it captures the sense of complexity and challenges associated with a tricky situation.