Which hardware device is responsible for building a data frame?

The Network Interface Card (NIC) is the hardware device responsible for building a data frame. This is because the NIC operates at the data link layer of the OSI model, where it takes packets from the network layer and wraps them in frames. This process includes adding necessary headers and trailers to the data to ensure it can be properly transmitted over the physical network medium.

Frames created by the NIC contain not only the data but also essential control information, such as the source and destination MAC addresses, which help ensure the proper delivery of the data to the intended recipient on a local network. This functionality is crucial for the communication between devices on the same local area network (LAN).

In contrast, while devices like switches and routers are involved in the networking process, their roles are focused on switching and routing packets rather than constructing data frames. A modem primarily serves to modulate and demodulate signals for transmitting and receiving data over telephone lines or cables but does not engage in the frame construction process. Therefore, the NIC is the correct answer as it directly handles the creation of data frames necessary for data transmission.