Which of the following exist as dimer?

Sulphur needs two electrons to complete its octet but it can expand its octet because of having empty d-orbitals. 

In sulphur monochloride (S-Cl), S is bonded to Cl and in this way, it cannot complete its octet. So, S atom of one S-Cl forms another bond with S atom of another S-Cl. In this way, S atom completes its octet. Hence, S-Cl usually exists in dimer form.