**Core Concept**
Chemotaxis is a fundamental process in cellular biology where cells move in response to chemical signals. This phenomenon is crucial in various biological processes, including immune responses, wound healing, and development. Chemotaxis involves the directed movement of cells towards or away from specific chemical cues.

**Why the Correct Answer is Right**
The main feature of chemotaxis is the directed movement of cells in response to a concentration gradient of chemical signals. This movement is facilitated by the reorganization of the cell's cytoskeleton, particularly the polymerization and depolymerization of actin filaments. The chemotactic response is also influenced by the activation of specific G-protein coupled receptors (GPCRs) that bind to the chemotactic molecules, triggering downstream signaling pathways.

Why Each Wrong Option is Incorrect
**Option A:** This option is incorrect because chemotaxis is a specific type of cell movement that is directed by chemical signals, rather than random or non-directed movement.
**Option B:** This option is incorrect because chemotaxis is a process that occurs in response to chemical signals, not physical forces like electromagnetism.
**Option C:** This option is incorrect because chemotaxis is a process that involves the movement of cells in response to chemical cues, not the movement of molecules through a membrane.

**Clinical Pearl / High-Yield Fact**
Chemotaxis plays a crucial role in the recruitment of immune cells to sites of infection or inflammation, making it a key aspect of the immune response. Understanding chemotaxis is essential for developing effective treatments for immune-related disorders.

**Correct Answer: D. Directed movement in response to a concentration gradient of chemical signals.**