To solve this problem, we need to understand the mechanisms by which ions move across a membrane against their concentration gradient. Let's analyze each option:$\newline \bullet$Osmosis:$\newline$ - Osmosis is the movement of water molecules through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration.$\newline$ - It does not involve the movement of ions against their concentration gradient.$\newline \bullet$Facilitated Diffusion:$\newline$ - Facilitated diffusion is a passive transport process where molecules move across the membrane through protein channels or carriers.$\newline$ - It occurs along the concentration gradient, not against it.$\newline \bullet$Passive Transport:$\newline$ - Passive transport involves the movement of substances across a membrane without the use of energy, along the concentration gradient.$\newline$ - It does not involve movement against the concentration gradient.$\newline \bullet$Active Transport:$\newline$ - Active transport is the process by which ions or molecules move across a membrane against their concentration gradient, using energy in the form of ATP.$\newline$ - This process requires specific transport proteins and energy input.$\newline$Therefore, the movement and accumulation of ions across a membrane against their concentration gradient can be explained by active transport.$\newline$The correct option is Option 4: Active Transport.