The answer is C: concrete.
One of the main lessons that classical empiricism, most notably Berkeley´s contributions, have left us is that ideas are nothing other than impressions, be that sense impressions or reflective sense impressions. For Berkeley, then, all ideas can be boiled down to concrete matters of fact, or what empiricists call experience, that is, information gathered by the senses. In this way, the only way to express abstract ideas is through concrete descriptions and definitions.