To solve this problem, we need to understand the role of the sodium-potassium pump,$\newline$also known as the $$Na$^+/K^+$ pump, which is a crucial membrane protein found in all animal cells.$\newline$This pump is responsible for maintaining the electrochemical gradient across the cell membrane.$\newline$The pump actively transports sodium ions (Na$^+$) out of the cell and potassium ions (K$^+$) into the cell.$\newline$Let's analyze the options to identify the ions involved in this pump:$\newline$• Option 1: $$Mg$^{2+}$ is not a constituent of the sodium-potassium pump.$\newline$• Option 2: $$K$^+$ is indeed a constituent of the sodium-potassium pump.$\newline$• Option 3: $$Fe$^{2+}$ is not involved in the sodium-potassium pump.$\newline$• Option 4: $$Ca$^{2+}$ is not a constituent of the sodium-potassium pump.$\newline$The sodium-potassium pump specifically exchanges $$Na$^+$ and $$K$^+$ ions across the cell membrane.$\newline$Therefore, the correct option is Option 2, $$K$^+.$