To solve this problem, we need to identify which of the given complexes is used as an anticancer agent.$\newline$Let's analyze each option:$\newline$• Option 1: $$mer-[Co(NH$_3$)$_3$Cl]$$ is a cobalt complex. Cobalt complexes are not typically used as anticancer agents.$\newline$• Option 2: $$cis-[PtCl$_2$(NH$_3$)$_2]$ is known as cisplatin. Cisplatin is a well-known anticancer drug used in chemotherapy.$\newline$• Option 3: $$cis-K$_2$[PtCl$_2$Br$_2]$ is a platinum complex, but it is not commonly used as an anticancer agent.$\newline$• Option 4: $$Na$_2$CoCl$_4$ is a cobalt complex and is not used as an anticancer agent.$\newline$Cisplatin (Option 2) is widely used in the treatment of various cancers, including testicular, ovarian, bladder, and lung cancers.$\newline$It works by binding to DNA and interfering with the cell's repair mechanism, leading to cell death.$\newline$Therefore, the correct option is Option 2: $cis-[PtCl_2({NH}_3)_2].$