Introduction:
In coordination chemistry, ligands are molecules or ions that bind to a central metal atom or ion to form a complex. Ligands can be classified into different types based on their charge and their ability to donate or accept electrons. Negative ligands are those that have a negative charge and donate electrons to the metal atom or ion.

Explanation:
Among the given options, the negative ligand is cyclopentadienyl (C5H5-). Let's discuss each option to understand why cyclopentadienyl is the correct answer.

a) Nitrosyl (NO+): Nitrosyl is a positive ligand since it has a positive charge (+).

b) Cyclopentadienyl (C5H5-): Cyclopentadienyl is a negative ligand because it has a negative charge (-). It donates its π-electrons to the metal atom or ion.

c) Carbonyl (CO): Carbonyl ligand is neutral and does not have a negative charge. It is a neutral π-acceptor ligand that donates its π-electrons to the metal atom or ion.

d) Thiocarbonyl (CS): Thiocarbonyl is neutral and does not have a negative charge. It is a neutral π-acceptor ligand that donates its π-electrons to the metal atom or ion.

Conclusion:
Among the given options, the negative ligand is cyclopentadienyl (C5H5-). It has a negative charge and donates its π-electrons to the metal atom or ion.