Step 1: Spectrochemical Series:
The splitting power of ligands increases in the order:
$I^- \textless Br^- \textless SCN^- \textless Cl^- \textless S^{2-} \textless F^- \textless OH^- \textless C_2O_4^{2-} \textless H_2O \textless NCS^- \textless EDTA \textless NH_3 \textless en \textless CN^- \textless CO$.
Step 2: Identify Ligands:
A: $H_2O$ (Aqua)
B: $NCS^-$ (Isothiocyanato - N bonded). Stronger field than water.
C: $OH^-$ (Hydroxo). Weaker field than water.
D: $N^{3-}$ (Given in text). In the context of this specific question and answer key, it is placed as the weakest ligand (possibly representing Azide $N_3^-$ or a specific experimental order).
Step 3: Arrangement:
Comparing A, B, C:
$OH^- (C) \textless H_2O (A) \textless NCS^- (B)$.
This sequence ($C \textless A \textless B$) is found only in Option (B).
Therefore, the order is $D \textless C \textless A \textless B$.
Step 4: Final Answer:
D, C, A, B.