The answer is D: Information processing.
Information processing is a very complex topic that, in a few words, refers to the process of how to make sense of changes in the universe, or in other words, the process of how an observer makes sense of information to which she is subjected. In this case, all the kinds of signs cited are forms of information that an observer interprets in order to understand changes in her surroundings that she should be aware of: from speed limits, to the flow of traffic or possible dangers ahead.
