To determine the oxidation number of platinum (Pt) in the complex ion PtCl62−, we need to consider the oxidation numbers of the other atoms in the ion and the overall charge. Chlorine (Cl) is a halogen and typically has an oxidation number of -1 when it's a part of a compound but not in a neutral elemental state. The complex ion PtCl62− has a total charge of -2 . There are six chlorine atoms, each contributing an oxidation number of -1 . This means the total contribution of the chlorines to the charge is 6×(−1)=−6. Now, let's set up an equation to solve for the oxidation number of platinum (Pt), which we'll call x: x+(6×−1)=−2 Solve for x : ‌x−6=−2 ‌x=−2+6 ‌x=4 Thus, the oxidation number of platinum in the complex ion PtCl62− is +4 . The correct answer is: Option B: +4