The milieu represents the surroundings of any place. For example, calmness is essential within the milieu of the hospital environment.
The metabolism (physiological activity) or components (a set of things) are going to depict a grammatical meaning in the above case.
Thus, the options (a), (d), and (c) are incorrect.
The milieu represents surroundings of any place. For example, calmness is essential within the milieu of the hospital environment.
Therefore, the correct option is –.