Explanation:

To connect multiple computers within a single Local Area Network (LAN), a **Switch** is the most common and efficient device. It operates at the data link layer (OSI $\text{Layer 2}$), forwarding data frames only to the specific destination port based on MAC addresses, which minimizes network traffic and collisions. * **Option A (Repeater):** A repeater extends network segment length by regenerating signals at the physical layer (OSI $\text{Layer 1}$), but does not join multiple computers into a LAN directly. * **Option B (Hub):** A hub also connects multiple devices in a LAN but operates at the physical layer (OSI $\text{Layer 1}$), broadcasting all data to all ports, leading to inefficiency and collisions. * **Option C (Router):** A router operates at the network layer (OSI $\text{Layer 3}$) and connects different networks (e.g., LANs to the Internet), not primarily computers within one LAN. * **Option D (Switch):** This is the correct answer. Switches intelligently connect devices within a LAN, providing dedicated connections and improving network performance.