Which of the following is a common selective estrogen receptor downregulator (SERD)?

A common selective estrogen receptor downregulator (SERD) is Fulvestrant. SERDs are designed to bind to estrogen receptors and downregulate their activity, leading to degradation of the receptor and reducing estrogen signaling. This is particularly beneficial in the treatment of hormone receptor-positive breast cancer, where estrogen can promote tumor growth. Fulvestrant serves this purpose effectively by not only blocking the receptor but also facilitating its degradation, leading to a more comprehensive inhibition of estrogen's effects on tumor cells.

The other options provided do not fulfill the criteria of a SERD. Tamoxifen and Raloxifene are selective estrogen receptor modulators (SERMs) that can act as estrogen receptor antagonists in some tissues while having estrogen-like effects in others. Competitive antagonists, while relevant in the context of cancer therapy, do not specifically refer to a class of drugs that includes SERDs. Therefore, Fulvestrant stands out as the appropriate choice.