The answer is D: the study of truth, knowledge, and the things of fundamental importance in life.
Philosophy has been defined, through the ages and, supposedly, according to Socrates´ own words, as the befriending of wisdom, that is to say, as an inclination towards the seeking of truth; the foundations of knowledge, its limits and its legitimacy; and the pursuit of value, beauty, and merit according to one´s own life in a social setting.