**Core Concept**
Active transport across the cell membrane is a process that moves molecules against their concentration gradient, requiring energy expenditure. This process is essential for maintaining cellular homeostasis, regulating the concentration of essential ions and molecules, and facilitating the uptake of nutrients.

**Why the Correct Answer is Right**
The correct answer involves the use of ATP (adenosine triphosphate) to drive the transport process. This is achieved through the action of transport proteins, such as pumps and channels, which use the energy from ATP hydrolysis to move molecules across the membrane against their concentration gradient. For example, the sodium-potassium pump (Na+/K+-ATPase) uses ATP to pump sodium ions out of the cell and potassium ions into the cell, maintaining the resting membrane potential and facilitating the proper functioning of nerve and muscle cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not specify the energy source or the type of transport process involved. Passive transport, on the other hand, does not require energy expenditure.
* **Option B:** This option is incorrect as it refers to a type of transport that does not involve energy expenditure. Facilitated diffusion is a type of passive transport that relies on the presence of transport proteins, but does not require energy.
* **Option C:** This option is incorrect as it is a type of transport that involves the movement of molecules down their concentration gradient, which is the opposite of active transport.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that active transport is crucial for maintaining cellular homeostasis and proper ion balance. Disorders affecting active transport, such as those involving the sodium-potassium pump, can lead to serious clinical consequences, including muscle weakness, cardiac arrhythmias, and seizures.

**Correct Answer:** C.