Final answer:
The RITS complex is responsible for mediating the RNAi silencing process by contributing to histone methylation and heterochromatin formation, leading to gene silencing.
Step-by-step explanation:
The protein complex that mediates the RNAi silencing process by inhibiting RNA polymerase through histone methylation and heterochromatin formation is the RNA-induced transcriptional silencing (RITS) complex, option B. It is not RISC, which is involved in degrading mRNA, nor is it mediator or RNase, which have other roles in the cell. The RITS complex plays a crucial role in the modification of histones and the formation of heterochromatin, which is a condensed form of DNA that silences gene expression by preventing access to the transcription machinery.