Step 1: Understanding the Concept:
Ligands are neutral, negative or positive species that donate electron pairs to a metal. Their charge must be known to find the charge on a complex.
Step 2: Detailed Explanation:
Ammonia \(\text{NH}_3\) is neutral. Methylamine \(\text{CH}_3\text{NH}_2\) is neutral. Hydrazinium \(\text{N}_2\text{H}_5^+\) carries a positive charge. Nitrate \(\text{NO}_3^-\) carries a negative charge.
Final Answer:
Nitrate is the negative ligand, option (D).
\[ \boxed{\text{NO}_3^-} \]