What is an environment called where all computers are physically attached to each other and data is transmitted at high speeds over short distances?

The environment described in the question is referred to as a Local Area Network (LAN). A LAN is characterized by the physical connection of computers within a limited geographical area, such as a single building or a campus. In this setup, computers are directly linked, allowing for high-speed data transmission over short distances. This is particularly advantageous for sharing resources like files and printers, and for facilitating communication among users in a confined location.

In contrast, other options represent different networking concepts. A network broadly encompasses any system of interconnected computers, which can vary in size and configuration. A modem is a device used to modulate and demodulate signals for data transmission over telephone lines or other mediums but does not refer to an environment or the arrangement of the computers themselves. A Wide Area Network (WAN) covers larger geographical areas and connects multiple LANs, usually over long distances, and is typically slower due to the greater distances involved and the technology used. Thus, a LAN is precisely the environment where computers are closely situated and communicate rapidly, distinguishing it from these other networking terms.